If it's honed, I would leave it for a couple of weeks and it might disappear. It happens to my Danby often. If not, Bellinzoni mangia macchia will pull out the oil, but it will leave an etch.

Try a poultice. Or a modern version: paper towels soaked with acetone, covered by Saran Wrap (to prevent the acetone from evaporating).

And then reseal!

I think there's a recipe for a poultice in one of the threads linked to in the "Read Me" thread (maybe the "Miscellaneous" topic?)

Here is a link that might be useful: New To Kitchens? Posting Pics? Read Me!

Just out of curiosity...is it a fat stain from the oil in the butter, or a color stain from the yellow coloring in the butter?

White Carrara can be quite porous-The bellonzoni storebought poultice is a sodium hypochlorite (bleach) formulation with clay. It works and will not etch marble.
You can make a homemade poultice using 30-40 % hydrogen peroxide and unscented baby powder mixed to a wet peanut butter consistency and placed over the stain(overlap) and covered with plastic overnight. After you pull the plastic dont touch again until the poultice is totally dry. Remove and see wha you get. You can use other methods as well such as acetone,alcohol ,bleach etc. For absorbant material you can use flour,baby powder,diatomaceus earth(used in pool filters) ,paper towels and even bleach. Our favorite is the peroxide. Wouldnt be a bad idea to check your countertop for porousity and apply a good sealer properly.
To check for porousity puddle up a palm sized puddle of water in several areas and leave there for 10 minutes or so then wipe the wate away and see if a darker mark remains. If so it needs to be sealed. Always remember sealers are a temporary means to inhibit staining agents from entering the stone.
Stu Rosen
www.mbstonecare.com
www.stoneshine.com

"paper towels soaked with acetone, covered by Saran Wrap "

And then you can try to remove the saran wrap turned into goo by the acetone.

you do not need anything as 'hot' as acetone for a fat stain, and it may drive the dye further into the stone.

Srosen, is Danby marble all that? Is it really that much of an improvement in durability over carrara and their like, to give up much of the beauty of those marbles? For the Danby they recommend rehoning the marble with Comet and a scrub pad after an etch. Would that also work with Carara and Statuary?

any porous marble or for that matter any porous stone can stain given the right circumstances and staining agent.
Even in this case sweetbabyjames wiped up the butter immediatley but it may have been soft butter and the wiping may have pushed some grease into the stone.It is possible that the stain may just disappear over a few days.
Washing with a alkaline cleaner may help to breakdown the grease as well. Try just putting flour or powder over the stain until you have the proper ingredients to poultice.
Always do the water test to check porosity of stone and see just how porous it is. Then make sure it is sealed.
You will be surprised that there are quite a few stones that will exhibit minimal porosity. Polished stones and many light travertines will have minimal porosity if any.
Also always remember if you do get a stain in most cases it can be removed.
As far as using comet or bon ami to remove an etch yes it may work under the right circumstances. First there are different levels of etching. If the etch is light enough (it will feel smooth to the fingertip)then yes on a honed surface using an abrasive cleaner and scrub pad may work.
But you have to be able to blend the repair into the rest of the countertop. If your surface is a high hone the abrasive cleaner will remove that etch but then you have to match the finish of the rest of the surface or rehone the whole piece or pieces.
Other etch marks that feel rough to the touch may require more agressive abrasives to remove them.
It would only make matters worse on a polished marble because once you remove the etch you now have a dull spot that still needs to be polished.There are products you can purchase that will remove etches and repolish the stone. They have their limitations as well but perform well on minor etching over small areas.
Improvements in durability over different stones? There is a difference in hardness in danby and statuary and there may be a difference in how they perform. I think it has more to do with the actual grade or quality of the stone itself. I also think the most important aspect of the whole stone experience is working with fabricators and installers that clarify and explain the pros,cons and complexities of the surfaces that you are purchasing or exploring. Besides that the client should be educated on the proper maintenance,ongoing care and refinishing in the future.
Stu Rosen
www.mbstonecare.com
www.stoneshine.com

Stu--You mentioned products that "there are products you can purchase that will remove etches and repolish the stone." Could you please tell us what those products are?

I appreciate you taking the time to reply to us.

Brickeyee, I appreciate the gobs of good, professional knowledge you bring to this forum. However, I do not believe you are correct in your post here. Saran wrap is just polyethylene, which is not soluble in acetone. (It is soluble at higher temperatures in some other, more aggressive ketones, and it is soluble at room temperature in some halogenated solvents.) I am traveling, and not able to test my assertions to my or your satisfaction at the moment.

Regarding driving the stain in: what a solvent and a poultice provide are (a) mobility of the constituent that you are trying to get rid of, and (b) a large number of binding sites for that constituent. By nature and necessity, some will indeed diffuse further in, but MOST will move to the paper towel or diatomaceous earth or talcum powder. This is simply statistical mechanics: there are many more places for the stain to be in the poultice than there are places for it to be in the stone. There is no magic substance that can "draw" the stain outwards. Only random motions (mobility) and binding.

Peroxides are a different story; here, you are trying to chemically modify the stain.

"Saran wrap is just polyethylene, which is not soluble in acetone."

Soluble and non-reactive are not the same thing.

There are better things that trying to guess what plastic will react with what solvent.

Yes-there are several companies that make these products.
We make one as well which works very good but they all have limitations. Black ,brown and serpentines(green marbles) are harder to polish than lighter marbles and limestones including travertine.The darker stones should possibly be refinished by a pro. Also if the etches feel rough they will need to be removed using more agressive form of abrasive.
It is also ideal for small areas and spot etches but for
larger areas say if someone or even a pet got sick it might not work as the acids from stomachs can be strong.
I have gotten calls from folks who have done a small bathroom marble floor however. There is a learning curve so if you do get these products get a spare piece of marble to practice on so you can get the hang of the technique.
Stu Rosen
www.mbstonecare.com
www.stoneshine.com

Neither high density nor low density polyethylene is considered compatible with acetone.

The 1 mil thickness is also not going to contain it well.

Try aluminum foil of you want to contain low boiling point liquids.