Preheat oven to 350 degrees F. Set aside 1/4 cup chocolate pieces to sprinkle on cookies. In microwave-safe bowl combine 1 cup of the chocolate pieces and 2 tablespoons milk. Microwave on 50 percent power (medium) 1 minute or until melted; stir once. Set aside.
In bowl beat butter and peanut butter with electric mixer until combine. Add sugar, soda, baking powder, and 1/2 teaspoon salt; beat to combine. Add eggs, 1/2 cup milk, vanilla, and melted chocolate; beat until combined. Beat in cocoa powder and flour until combined. Stir in remaining chocolate pieces. Drop by rounded teaspoons 2 inches apart onto ungreased cookie sheets. Sprinkle with reserved chocolate. Bake 9 minutes or until tops are set. Cool on sheets 1 minute. Transfer to wire racks; cool completely. To assemble, spread flat sides of half the cookies with Filling; top with remaining cookies.
In bowl beat butter, peanut butter, marshmallow creme, and powdered sugar with electric mixer until combined. If needed, beat in 2 tablespoons milk until spreading consistency. Makes 30 servings.