The one at the link below uses a Bundt pan. I have a recipe that calls for a 8x12" pan.

CHOCLATE PECAN UPSIDE-DOWN CAKE
Pecan Topping:
4 TBL unsalted butter, melted
1/2 cup packed brown sugar
1/2 cup light corn syrup (Karo)
1 1/2 cups coarsely chopped pecans

Cake:
2 cups flour
1 tsp baking powder
1/2 tsp salt
8 TBL butter, softened (1 stick)
1 1/2 cups sugar
3 eggs
3 sq (3 oz) unsweetened chocolate, melted
3/4 cup sour cream
1 tsp vanilla

For the topping: In a small bowl, combine the butter, brown sugar & syrup. Pour into a greased 8x12" baking pan. Scatter pecans evenly on top & set aside.
For the cake: Preheat oven to 350°. Whisk together the flour, baking powder & salt 7 set aside. Cream the butter; gradually add the sugar. Beat until light & fluffy. Beat in the eggs one at a time. Stir in the melted chocolate. Combine the sour cream & vanilla. Add to the batter alternately w/the dry ingredients. Pour over the topping in the pan. Bake 45 minutes or until a tester inserted in the center of the cake comes out clean. Loosen the edges with a sharp knife. Invert onto a cake plate immediately. serve w/whipped cream of vanilla ice cream.~~Scarsdale Entertains

Here is a link that might be useful: Chocolate Nut Upside-Down Cake

Here's one I've tried & everyone really liked, I didn't use a bundt pan though, & I don't remember adding the marshmallows either:
Upside Down German Chocolate Cake Frosting:
1 ¼ c water
¼ c butter
1 c brown sugar
1 c coconut
2 c mini marshmallows
1 c nuts
Cake: 4 oz semi sweet chocolate
½ c water
2 ½ c flour
1 ½ c sugar
½ t salt
1 t soda
1 c sour cream
½ c soft butter
1 t vanilla
3 eggs
In saucepan heat 1 ¼ c water and ¼ c butter til melted. Pour in 13x9 pan. Stir in brown sugar and coconut; sprinkle marshmallows and nuts evenly over top. In saucepan, over low heat, melt chocolate with ½ cup water. Combine chocolate mix with remaining cake ingredients. Beat 3 min at med speed. Spoon batter over marshmallow mixture. Bake at 350 for 45-55 min. Or use cake mix.