COMBINE flour, CALUMET Baking Powder, and salt in a medium bowl, then set aside.
HEAT and stir chocolate over low heat until melted in a heavy small saucepan, stirring constantly. Set aside.
BEAT butter with an electric mixer on medium speed for 30 seconds in a medium bowl. Then, add in sugar and beat until fluffy. Beat in melted chocolate, egg, and vanilla.
GRADUALLY beat in the flour mixture. Divide dough into four equal portions. Wrap each portion in plastic wrap or waxed paper. Freeze for 20 to 30 minutes or until dough is firm enough to handle. (Or chill in the refrigerator for 60 minutes.)
PREHEAT oven to 350. Line cookie sheets with parchment paper; set aside. Removing one portion of dough from freezer at a time, roll each portion into a 10-inch roll. To roll, 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 the freezer for a few minutes. Wrap and freeze for 5 minutes more. Cut rolls into 1/2-inch slices. Place slices 1 inch apart on prepared cookie sheets. Bake for 9 to 10 minutes or until edges are set. Cool on cookie sheet for 2 minutes. Gently remove from sheets (cookies will be brittle) and transfer to a wire rack to cool completely.
MELT chocolate and butter over low heat in a medium saucepan, stirring frequently. Cool for 5 minutes. Stir in BREAKSTONE'S Sour Cream. Gradually add powdered sugar until spreadable.
SPREAD frosting on the bottom sides of half of the cookies, using ½ teaspoon for each cookie; top with remaining cookies, bottom sides down, to make sandwiches.