Shirley, This recipe is made from scratch not a mix and it is for a bar not a cake but I think you could top your mix with marshmallows as in this recipe and make the Choco Topping. I hope this is helpful.

MISSISSIPPI MUD BARS (AKA. BARGE BARS)
a marshmallow fudge brownie

1/3 cup butter
1-ounce square unsweetened chocolate
1 cup sugar
3/4 cup flour
1/2 tsp. salt
1/2 tsp. baking powder
2 eggs
1 tsp. vanilla
1/2 to 1 cup chopped nuts
16 large marshmallows, halved

Melt butter and chocolate over low heat. Remove from heat. Stir in sugar. Sift together flour, salt and baking powder. Add to chocolate mixture. Beat in eggs. Stir in vanilla and nuts. Pour into greased 11" x 7" pan. Bake at 350 deg. for 35 minutes. Remove from oven. Immediately top with marshmallows. Return to oven for 3 minutes, or until marshmallows are soft and easy to spread. Remove from oven and gently spread marshmallows over top. Cool

CHOCO TOPPING:
1 (1 ounce) square unsweetened chocolate
1/2 cup brown sugar
1/4 cup water
3 Tbsp. butter
1 tsp. vanilla
1/2 tsp. salt
1 2/3 cups confectioners sugar

Boil chocolate, sugar and water for 4 minutes. Cool. Add butter. Stir in rest of ingredients. Spread over marshmallows.

Makes 2 dozen