In a medium bowl, combine flour, baking powder, and salt; set aside.
In a heavy small saucepan heat the chocolate over low heat until melted, stirring constantly. Set aside.
In a medium bowl, beat butter with electric mixer on medium speed 30 seconds. Beat in sugar until fluffy. Beat in melted chocolate, egg, and vanilla until combined.
Gradually beat in the flour mixture. Divide dough into four equal portions. Wrap each portion in plastic wrap. Freeze for 20 to 30 minutes or until firm enough to handle. (Or refrigerate for 60 minutes.)
Preheat oven to 350 degree F. Removing one portion of dough from freezer at a time, roll each portion into a 10-inch-long roll*. Cut each roll crosswise into 1/4-inch pieces. Place slices 1 inch apart on ungreased cookie sheets. Bake for 9 to 10 minutes or until edges are set. Let stand on cookie sheet for 2 minutes. Gently remove from sheets (cookies will be brittle) and transfer to wire rack to cool completely.
If desired, spread 1/2 teaspoon of frosting on half of the cookies; top with remaining cookies. Serve the same day. To store, place plain cookies in airtight containers at room temperature up to 3 days or freeze in freezer containers up to 3 months. Thaw, then fill, if desired. Makes 144 tiny cookies or 72 tiny sandwich cookies.
*Rolling Tip: It helps to place the dough on a sheet of waxed paper and use the paper to help shape the roll. If dough becomes too sticky, return to freezer for a few minutes.
Milk Chocolate Sour Cream Frosting: In a medium saucepan melt 3 ounces chopped milk chocolate and 2 tablespoons butter over low heat, stirring frequently. Cool for 5 minutes. Stir in 1/4 cup dairy sour cream. Gradually add 1 to 1-1/4 cups sifted powdered sugar until spreadable.