1 cup softened butter, no substitutes
2 cups all-purpose flour
1 3/4 cups sugar
1 teaspoon soda
1 teaspoon pure vanilla
2 eggs
1/2 cup buttermilk
1 cup Dr. Pepper
1 1/2 cups miniature marshmallows
ICING:
1/2 cup (1 stick) butter
3 tablespoons cocoa
1/3 cup Dr. Pepper
1 box powdered sugar
1 cup chopped pecans

Preheat oven to 350 degrees.
Combine all ingredients, except marshmallows, in large mixing bowl; blend at low speed until thoroughly mixed, then fold in marshmallows. Pour batter into greased 9- by 13-inch glass baking dish. Bake at 350 degrees for 35 minutes or until toothpick comes out clean; cool 15 minutes and then spread icing on top.
Icing: Combine butter, cocoa, Dr. Pepper and powdered sugar. Beat until smooth, then stir in pecans and spread over cake.