This reciipe is not made with a cake mix but perhaps if you use the cake portion from Kelly~WA and this frosting (made with brown sugar)you will come up with what you are looking for. Hope it helps

BARGE BARS (MISSISSIPPI MUD BARS)
(a marshmallow fudge brownie)
Makes 2 dozen

1/3 cup butter
1 oz. 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

Choco Topping:
1 oz. 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

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 11x7 inch 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: Boil chocolate, sugar and water 4 minutes. Cool. Add butter. Stir in rest of ingredients. Spread over marshmallows.