1½ cups all-purpose flour
Pinch of kosher salt
½ cup unsweetened cocoa power, sifted
8 ounces (16 tablespoons) cold unsalted butter, cut into ½-inch pieces
½ cup granulated sugar
Flour for dusting
1 cup mascarpone cheese
1 tablespoon granulated sugar
¼ teaspoon vanilla extract
To make the chocolate shortbread:
In a bowl, stir together the flour, salt, and cocoa powder and set aside. Combine the butter and sugar in the bowl of a stand mixer fitted with the paddle attachment and beat on low speed until the butter and sugar begin to incorporate, about 15 seconds. Add the dry ingredients and continue to mix until the dough comes together, about 3 minutes. It will look dry just before it comes together.
Line 2 baking sheets with parchment paper. On a lightly floured work surface, roll out the dough ¼ inch thick. Using a 2-inch star cutter, cut out scraps, reroll, and continuing cutting out cookies until you have 36 cookies. Place the cookies on the prepared baking sheets, spacing them 1 inch apart. Refrigerate until firm, at least 1 hour.
Preheat the oven to 300°F. Bake the shortbread until firm, about 35 minutes. At the midway point, switch the baking sheets between the racks and rotate them 180 degrees to ensure even baking. Let cool on the baking sheets to room temperature.
To make the filling:
In a small bowl, stir together the mascarpone, sugar, and vanilla until smooth. Turn 18 of the cookies bottom-side up on a work surface. Using a table knife or small icing spatula, spread about 1 tablespoon of the filling on the bottom of each cookie. Top with remaining cookies, bottom-side down Planning Ahead The shortbread dough may be made and cut out a week in advance. Wrap tightly and refrigerate or freeze.
The cookies may be baked 2 days in advance. Store in an airtight container at room temperature. The mascarpone cream may also be made 2 days ahead. Cover and refrigerate, but bring to room temperature before assembling the sandwiches. The cookies should be assembled the day you are going to serve them and keep covered and refrigerated until serving.