3 sticks unsalted butter, at room temperature

3 cups firmly packed light brown sugar

3 eggs

1 tbsp vanilla extract

3 1/2 cups all-purpose flour

1 cup plus 2 tbsp unsweetened cocoa powder

1 ½ tsp. baking soda

1 ½ tsp. cream of tartar

¾ tsp. salt

¼ cup coarse sugar, or as needed

In a large bowl, using an electric mixer, beat the butter and brown sugar on medium speed until creamy. Add the eggs and vanilla and beat until smooth. In another bowl, stir together the flour, cocoa powder, baking soda, cream of tartar and salt. Reduce the speed to low, add the flour mixture to the butter mixture and beat until blended.
Divide the dough into 3 equal pieces, pat each into a flat disk and wrap separately in plastic wrap. Refrigerate 1 disk for at least 1 hour to use now, and store the remaining 2 disks for future use (see note above).
Preheat an oven to 350°F (180°C). Line 2 baking sheets with parchment paper.
On a work surface, place the disk between 2 sheets of plastic wrap and roll out about 1/8 inch (3 mm) thick. Use a cookie cutter to shape the dough. Sprinkle the cookies with the coarse sugar.
Bake the cookies until the edges are crisp, 8 to 10 minutes. Let the cookies cool.