Add the flour, sugar, sour cream, baking powder, and salt to Basic Recipe dough. Beat to combine. add the melted chocolate and beat until completely incorporated. (Dough will be soft.) Shape the dough into a 12-inch log, wrap in plastic wrap and refrigerate until firm, at least 2 hours or overnight.
Preheat the oven to 350°F. Line 2 baking sheets with parchment paper.
Slice the chilled log into 30 slices. Bake for 9 to 11 minutes. Cool completely on a wire rack. Using a star shaped stencil, dust the cookies with powdered sugar just before serving.
In a medium bowl combine the flour, baking soda, and salt.
In a separate large bowl beat the butter and sugars with an electric mixer until light and fluffy, about 2 minutes. Add the eggs, one at a time, beating well after each addition. Beat in the vanilla. Gradually add the flour mixture, beating until just combined.
Divide the dough in half, and proceed with a cookie recipe. (Each recipe uses half of the Basic Recipe.) To save the dough for later use, shape each half into a disc, wrap in plastic, and freeze for up to a month.