1. Beat butter, sugar, egg and vanilla in large bowl until well blended. Stir together flour, cocoa, baking powder and salt; add to butter mixture, mixing until well blended. Divide dough into quarters; wrap tightly. Refrigerate 2 to 3 hours.
2. Heat oven to 325°F. On lightly floured surface, roll out dough, one-fourth at a time, to 1/8-inch thickness. Cut with pumpkin, ghost or cat shaped cookie cutters. Place cookies 1 inch apart on ungreased cookie sheet.
3. Bake 8 to 10 minutes or until firm. Remove from cookie sheet to wire rack; cool completely. Pair cookie bottoms with matching tops. Decorate cookie tops with DECORATOR FROSTING, if desired.
4. Place two chocolate pieces and marshmallow half on flat side of 1 cookie bottom; place on paper towel. Microwave at MEDIUM (50%) in 10 second intervals until marshmallow puffs. Top with matching cookie top; press down slightly. Repeat with remaining cookies. Makes about 18 filled cookies.
DECORATOR FROSTING: Combine 3 tablespoons water and 1 tablespoon meringue powder in small bowl. Add 1-1/4 to 1-1/2 cups powdered sugar and 1/8 teaspoon vanilla extract; beat on high speed of mixer until stiff.