24 Tbs. (3 sticks) unsalted butter, at room temperature
3 cups firmly packed light brown sugar
1 Tbs. vanilla extract
3 1/2 cups all-purpose flour
1 cup plus 2 Tbs. unsweetened cocoa powder
1 1/2 tsp. baking soda
1 1/2 tsp. cream of tartar
1/4 tsp. salt
4 Tbs. 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.
Turn out the dough onto a floured work surface. 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. 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 thick. Using a 2-inch star-shaped (or other) cookie cutter, cut out cookies. Transfer them to the prepared sheets, spacing them 1 1/2 inches apart. Gather the scraps, reroll and cut out more cookies. Sprinkle the cookies with the coarse sugar.
Bake the cookies until the edges are crisp, 8 to 10 minutes. Transfer the baking sheets to wire racks and let the cookies cool briefly on the sheets, then transfer the cookies to the racks and let cool completely. Makes about 40 cookies; makes 3 cookie dough disks total.
Can be used to make star cookies or for ice cream sandwiches.