Preheat the oven to 350?°F (175?°C).
In large bowl, beat 1/2 cup of butter, 1 cup of sugar and 1 teaspoon of vanilla until creamy. Add 3 eggs, one at a time, beating well after each addition.
Stir together 3/4 cup of all-purpose flour, 1 teaspoon of cocoa powder, 1/2 teaspoon of baking powder and a pinch of salt and then blend it into the butter mixture.
Stir in a cup of chopped pecans. Spoon the batter into a greased 13-by-9-by-2-inch baking pan.
Bake for 15 to 18 minutes or until the top springs back when touched lightly in center.
While the cake is baking, make the frosting.
Beat 6 tablespoons of softened butter, then blend in 2-2/3 cups of powdered sugar, 1/2 cup of cocoa powder and 1/3 cup of milk.
Beat to a spreading consistency (additional milk may be needed).
Blend in 1 teaspoon of vanilla extract.
Remove the cake from oven; immediately cover the top with marshmallows.
Return the cake to the oven for 2 to 3 minutes or until marshmallows are soft.
As soon as it comes out of the oven again, spread the frosting over the marshmallow layer.
Cool thoroughly before cutting the cake into squares.
