I have a recipe for Mississippi Mud Cake, but it is baked in
a 9 x 13-inch pan, so it can be cut into bars. I'm not sure
about the icing, but maybe this will help.

Mississippi Mud Cake

2 sticks margarine
1/2 cup cocoa
4 eggs, beaten
1 cup sugar
1 1/2 cups all-purpose flour
Dash of salt
1 teaspoon vanilla
1 cup walnuts
Miniature marshmallows

ICING

1/3 cup cocoa
1/2 stick margarine
1/2 cup milk
1 box powdered sugar

Melt margarine and cocoa together. Add beaten eggs.
Blend in remaining ingredients. Bake at 350 degrees for
30 to 35 minutes in a 9x13-inch pan. Immediately after
taking from oven, cover top with miniature marshmallows.

ICING: Melt cocoa and margarine. Add milk and
powdered sugar. Blend together and pour over cake while
it is still hot.

NOTE: Make the icing the last few minutes the cake is
baking.

A DIFFERENT ICING RECIPE: