Preheat oven to 350 degrees. Sift together flour, baking powder, and salt into a medium bowl. Put butter and granulated sugar in the bowl of an electric mixer fitted with the paddle attachment, mix on medium speed until pale and fluffy, about 4 minutes. Add eggs and creme de cassis, and mix until combined. Add flour mixture, and mix on low speed until smooth.
Divide dough in half, and wrap each half in plastic; refrigerate until cold, about 1 hour. Working with 1 half at a time, roll out dough on a lightly floured surface to 1/4 inch thick, and cut into 3-inch squares. Space 2 inches apart on baking sheets lined with parchment paper, and sprinkle with sanding sugar. Reroll scraps; repeat with remaining dough.
Bake cookies until golden brown all over, 12 to 15 minutes. Let cool on sheets on wire racks. Cookies can be stored in an airtight container at room temperature up to 3 days.