1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Beat butter, sugar and vanilla in large bowl until creamy. Add eggs, one at a time, beating well after each addition. Stir together flour, cocoa, baking powder and salt; blend into butter mixture. Stir in pecans. Spoon batter into prepared pan.
3. Bake 15 to 18 minutes or until top springs back when touched lightly in center. Meanwhile, prepare frosting. Remove cake from oven; immediately place marshmallows over top. Return cake to oven 2 to 3 minutes or until marshmallows are soft. Gently spread marshmallows over cake; immediately spread frosting over marshmallow layer. Cool thoroughly before cutting cake into squares. 12 to 15 servings.
ONE-BOWL BUTTERCREAM FROSTING
6 tablespoons butter or margarine, softened
2-2/3 cups powdered sugar
1/2 cup HERSHEY'S Cocoa
1/3 cup milk
1 teaspoon vanilla extract
Beat butter in medium bowl. Blend in powdered sugar and cocoa alternately with milk; beat to spreading consistency (additional milk may be needed). Blend in vanilla. About 2 cups frosting.