Chocolate-Marshmallow Ribbon Cake

1/4 cup unsweetened baking cocoa

1 teaspoon baking powder

1/4 teaspoon salt

3 eggs

1 cup granulated sugar

1 teaspoon vanilla

Powdered sugar

2 egg whites

1 1/2 cups granulated sugar

1/4 teaspoon cream of tartar

1 tablespoon light corn syrup

1/3 cup water

16 large marshmallows, cut into fourths

1 oz unsweetened baking chocolate

1 teaspoon butter or margarine

1 cup powdered sugar

5 teaspoons boiling water