DOUBLE-CHOCOLATE SANDWICH COOKIES

Steps:

• Make dough:
• Whisk together flour, cocoa powder, baking powder, and salt.
• Beat butter and sugar with an electric mixer until pale and fluffy, then beat in yolk and vanilla. At low speed, mix in flour mixture in 3 batches just until a dough forms. Divide dough in half and form each piece into a 6-inch square, then chill, wrapped in plastic wrap, until firm, 2 to 3 hours.
• Make ganache while dough chills:
• Bring cream and corn syrup just to a simmer in a small heavy saucepan, then stir into melted chocolate. Stir in butter and vanilla until smooth. Cover surface with parchment paper and chill, stirring occasionally, until very thick, about 30 minutes.
• Cut and bake cookies:
• Preheat oven to 350°F with rack in middle. Butter 2 large baking sheets.
• Roll out 1 piece of dough between sheets of parchment paper into a 14- by 10-inch rectangle (1/8 inch thick). Slide dough in parchment onto a tray and freeze until dough is firm, about 10 minutes. Repeat with remaining dough.
• Cut out as many rounds as possible from first chilled square with cutter, reserving and chilling scraps, then quickly transfer cookies to a buttered baking sheet, arranging them 1/2 inch apart. (If dough becomes too soft, return to freezer until firm.)
• Sprinkle half of cookies with decorative sugar (if using), then bake cookies until baked through and slightly puffed, 10 to 12 minutes. Cool on sheet on rack 5 minutes, then transfer to rack to cool completely (cookies will crisp as they cool).
• Make more cookies with remaining dough and scraps (reroll only once).
• Assemble sandwich cookies:
• Beat ganache with an electric mixer at high speed just until light and fluffy. Transfer to a sealable plastic bag (snip off 1/8 to 1/4 inch from 1 corner with scissors). Pipe ganache onto flat sides of plain cookies, then top with sugared cookies to make sandwiches. Chill, layered between sheets of parchment, in an airtight container until filling is set, at least 1 hour.