MAKE THE DOUGH
• Sift the flour, cocoa powder, and salt together into a bowl. Set aside.
• Put the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed just until combined. Add the vanilla extract.
• Switch the mixer to low speed and add the dry ingredients in 3 additions, pulsing the mixer to incorporate each addition before adding the next one. The dough will look dry.
• Turn the dough out onto a lightly floured work surface. Knead a few times just until it comes together. Flatten into a disk, wrap in plastic wrap, and refrigerate until firm, at least 3 hours or up to 2 days.
BAKE THE COOKIES
• Preheat the oven to 325F. Line the bottoms of two 12-by-18-inch sheet pans with parchment paper.
• Remove the dough from the refrigerator and unwrap it. On a lightly floured work surface, roll out the dough 1/8" thick. Using a 1 1/2-inch round cookie cutter, cut out as many rounds as possible. Re-roll the scraps only once, using less flour on the work surface to prevent toughness, and cut again. You should have 60 rounds in all. Place the rounds on the prepared pans, spacing them 1/2 inch apart.
• Bake on the middle shelves of the oven, rotating the pans 180 degrees halfway through the baking time, until the tops are lightly cracked and hold a slight indentation when pressed with a fingertip, about 10 minutes. While cookies are still warm, press your fingertip into the center of each cookie to make an indentation the size of a quarter. Let cool completely on the pans on wire racks. (If the cookies are too warm, the truffles will melt and ooze out of the indentation).
• When cookies are cool, spread just enough Extra-Bitter Chocolate Sauce on the bottom of each Signature Truffle to make them stick and press into the indentation on each cookie.
• Store in an airtight container at room temperature.
