2 sticks cold unsalted butter, cut into 1/2-inch dice
1 teaspoon pure vanilla extract
1 ounce unsweetened chocolate, melted and cooled slightly
In a food processor, pulse the flour with both sugars and the cornmeal, cocoa powder, salt and baking soda. Add the butter and vanilla; pulse until the mixture resembles moist sand. Add the melted chocolate and pulse just until the dough comes together. Pat the dough into 2 disks, wrap in plastic and refrigerate until chilled, about 2 hours.
Preheat the oven to 325°. Line 2 large baking sheets with parchment paper. Roll out each dough disk 1/4 inch thick between 2 sheets of parchment paper. Using a 2 1/2-inch round cookie cutter, stamp out cookies as close together as possible. Transfer the rounds to the prepared baking sheets 1 inch apart. Gather and reroll the scraps and stamp out more cookies.
Bake the cookies for about 18 minutes, until the tops are firm. Let cool completely on the baking sheets before serving.