This Prune Cakes does not fit the description of what you are looking for, but it is none-the-less delicious!! Thought since you have someone in the family fond of prunes, perhaps you would like it for future use.

Prune Sour Cream Cake
Source: "from the back of Sunsweet Prune box"
1½ cups Sunsweet Prunes (use already pitted, soft
prunes)
1 tablespoon grated lemon rind
2 cups sifted flour
1 teaspoon baking powder
1 teaspoon baking soda
½ teaspoon salt
1 cup soft butter
1 cup granulated sugar -- * I use ¾ cup
2 large eggs
1 cup sour cream
1 teaspoon vanilla extract
½ cup firmly packed light brown sugar
1 teaspoon cinnamon
½ cup chopped English walnuts
Pour boiling water over prunes. Let stand 15 minutes. (Skip this step if using the soft pitted kind.) Drain, pit, and dice prunes. Add lemon and set aside.
Grease and flour 9" tube pan. Sift together flour, baking powder, baking soda, salt; remove 1/4 cup and toss with prunes.
Cream butter and sugar until fluffy. Beat in eggs, one at a time. Slowly beat in flour mixture, alternnately with
sour cream and vanilla, beginning and ending with flour.
Fold in prunes. Combine brown sugar, cinnamon and nuts. Turn 1/3 batter into pan. Sprinkle with 1/3 brown
sugar mixture, repeat layering twice.
Bake at 350° F. 55 minutes or until done. Cool in pan on rack for 10 minutes. Remove from pan.
Description: "Coffee Cake Type. Tried and True! Very, very good!!"

This is from The Boston Cooking School Cook Book 1931

Prune Almond Filling

White Mountain Cream (will list later)
1/2 cup prunes, stoned and cut in pieces
1/3 cup almonds, blanches and cut into pieces
When frosting is ready to spread, add prunes and almonds.

If I remember, my mom and grandma always made boiled frostings

White Mountain Cream Boiled frosting I

1 cup sugar
1/2 cup water
1/2 teaspoon cream of tartar or 1/2 tablespoon lemon juice
1 egg white, beaten until stiff

Put sugar, water, and cream of tartar in pan, stir until sugar is dissolved, and bring to boiling point. Add 3 tablespoons sirup to beaten whites, beating constantly after adding each spoonfull. boil sirup to 240F or until it spins a long thread. Pour gradually on egg white, beating constantly. Flavor as desired.

Years ago, they did not have the prepared stuff and would not use powder sugar. Boy I miss those fluffy frostings.
I checked the book for prune cakes, and it appeared that they did not put prunes into the cake, but only into the frosting. Guess, a person could get too many prunes===
You could also check any German receipes as they use alot of prunes. My friend made a upside prune cake and it was not too sweet, buy very tasty.

Here is where you can find a layer cake with prune filling.
Just stumbled onto it while searching with Google.

Maybe if you borrow portions of different recipes, you can come somewhat close to what you want to achieve.

Here is a link that might be useful: Icelandic Layer Cake with Prune Filling

Prune Spice Cake
1 cup boiling water
1 cup uncooked prunes, pitted and cut up
2 cups sifted flour
1 1/2 cups sugar
1 teaspoon salt
1 1/4 teaspoons baking soda
1 teaspoon each cinnamon, nutmeg, cloves
1/2 cup oil
3 eggs, unbeaten
1 cup chopped nuts
Pour boiling water over prunes, let stand 2 hours. Sift dry ingredients into large mixing bowl. Add prune mixture, oil, eggs and nuts. Blend thoroughly about 1 minute. Beat 2 minutes at medium speed on mixer.
Pour into a greased and floured 13 x 9 x 2 inch pan. Sprinkle batter with Streusel topping.
Bake in oven 350 degrees 45 to 50 minutes. May also be baked in cupcake lined pans, makes 12 cup cakes. May omit the Streusel topping. Can also be baked in two 9 inch round pans and frosted.
Streusel Topping
Mix 1/2 cup sugar
2 Tablespoons Flour
2 Tablespoons butter or margarine
Mix until crumbly and sprinkle on cake.
I hope this is the recipe you are looking for.

Sue

Here's another......Golden Beauty Prune Layer Cake.

Calls for Mocha Frosting, but that can be changed!

Here is a link that might be useful: Prune Layer Cake