Dark- and White-Chocolate Shortbread Hearts

Dark-chocolate shortbread: In a bowl, whisk together flour, cocoa powder, espresso powder, and salt. Beat butter on medium speed until creamy; add confectioners' sugar and beat until combined, then beat in vanilla. Add flour mixture and beat just until combined. Wrap dough in plastic, flatten to a 1-inch-thick disk, and refrigerate until firm, at least 1 hour and up to 3 days.

White-chocolate shortbread: In a bowl, whisk together flour and salt. Beat butter with white chocolate on medium speed until creamy; add confectioners' sugar and beat until combined, then beat in vanilla. Add flour mixture and beat just until combined. Wrap dough in plastic, flatten to a 1-inch-thick disk, and refrigerate until firm, at least 1 hour and up to 3 days.

Remove both disks from refrigerator; let stand 10 minutes. Roll out dark-chocolate dough 1/8 inch thick between two pieces of floured parchment. Stamp out heart shapes with largest cookie cutter; transfer to a parchment-lined baking sheet. Gather dough scraps, reroll, and stamp out more heart shapes with largest cutter. Freeze cutouts until firm, about 15 minutes. Repeat with white-chocolate dough.

Stamp out smaller hearts from larger ones, starting with second-largest cutter and working down to smallest cutter; transfer cutouts to parchment-lined sheets. Freeze cutouts until firm, about 15 minutes.

Preheat oven to 325 degrees with racks in upper and lower thirds. Starting with smallest cutouts and working up to largest, fit together dark- and white-chocolate cutouts like puzzle pieces. (It's okay if dough cracks or breaks in places; simply press back together as necessary.) Arrange completed hearts 1 inch apart on fresh parchment-lined baking sheets. Freeze again until firm. Working in two batches, bake cookies, rotating sheets and rack positions halfway through, until dry and firm, 15 to 18 minutes. Transfer cookies to wire racks; let cool completely. Cookies can be stored in an airtight container at room temperature up to 1 week.