Chocolate Cookie Dough

• Unsalted butter, 1½ cups (12 oz/375 g), at room temperature

In a large bowl, using an electric mixer on medium speed, beat the butter and brown sugar 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. Add the dry ingredients to the wet ingredients and mix on low speed until blended. Turn out the dough onto a lightly floured work surface. Divide int