Cream butter; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition. Combine flour, baking powder, salt, and cocoa; add to creamed mixture. Stir in vanilla and pecans.
Spoon batter into a greased 13- x 9- x 2-inch glass baking pan. Bake at 325° for 15 to 18 minutes or until top is barely soft to the touch.
Remove cake from oven and cover top with marshmallows
Return to oven for 2 minutes or until marshmallows are soft. Spread marshmallows over cake and immediately cover with Chocolate Frosting. Let frosting harden before cutting the cake into squares.