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 13x9x2-inch glass baking pan. Bake at 160ºC or 15 to 18 minutes 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. Yield: one 13x9 inch cake. FROSTING: Cream butter; add cocoa, mixing well. Gradually beat in powdered sugar, adding warm milk as necessary, until spreading consistency. Stir in vanilla. Spread immediately over warm marshmallows. Yield: enough for one 13x9 inch cake.