This dough is very easy to work with once it's chilled, and I say that as someone who has little skill or patience for this kind of thing. Chocolate sugar cookies are a welcome variation on a terrific theme, and these are excellent ones.
In a large bowl or the bowl of a stand mixer fitted with the paddle, beat together the sugar, butter, baking powder, vanilla and salt until lightened and fluffy, about 5 minutes, scraping down bowl as necessary. Beat in egg until well combined.
Into a medium bowl, sift together the cocoas and flour. Beat into wet ingredients all at once, just until well combined. Refrigerate dough for 30 minutes. (Dough will keep in fridge for 24 hours if you want to prepare it the day before.)
Preheat oven to 375°F with two racks close to the center. Line two baking sheets with parchment.
Divide dough into two discs. Sprinkle with cocoa. Roll out each disc until dough is 1/8" thick and cut into shapes with cookie cutters. Transfer cookies to baking sheets, leaving space between cookies.
Bake cookies for 8 to 10 minutes, until edges begin to firm. Cool on pan for a couple of minutes and then completely on a rack before decorating.
Adapted from King Arthur.