Melt the chocolate in the top of a double boiler over barely simmering water; set aside to cool slightly. In a small bowl, whisk together the flour, salt, and baking powder; set aside. In a medium bowl, with an electric mixer on medium speed, cream the butter and sugar together. Mix in the egg and vanilla extract. On low speed, slowly add the flour mixture to the butter mixture and combine well.

Place one half of the dough on a sheet of waxed paper. Stir the cooled melted chocolate into the remaining half of the dough. Blend until the color is uniform. Scrape the chocolate dough onto the waxed paper alongside the vanilla dough and press the two together to form a ball. Knead just long enough to achieve a marbleized effect. Shape the dough into a 1 by 12-inch log. Wrap tightly in waxed paper or plastic wrap and twist the ends closed. Chill for 2 to 3 hours or until firm.

Preheat the oven to 375°F. Lightly grease the baking sheets. Cut the chilled dough into 1/4-inch-thick slices and place 1 inch apart on the prepared baking sheets. Bake in the center of the oven for about 8 to 9 minutes, or until the cookies are set but not browned. Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely. Store in an airtight container. Well-wrapped cookies may be frozen.