In a bowl, cream butter and sugar.
Add eggs and vanilla.
Beat until fluffy.
Combine flour, baking powder, salt and cocoa.Add to creamed mixture
.Stir in nuts.Spread in a greased jelly roll pan.Bake at 350 for 15-18 minutes.
Sprinkle marshmallows evenly over cake.Return to oven for 2-3 minutes.
Using a knife dipped in water, spread the melted marshmallows over cake.Cool.