1. In a bowl, using an electric mixer, beat the butter and sugar on medium speed until blended, about one minute. Add the egg and vanilla and beat until combined. Turn off the mixer and scrape down the bowl with a rubber spatula. Sift the flour, cocoa, baking soda, and salt into a separate bowl, then add to the butter mixture. Mix on low speed just until blended. Cover the bowl with plastic wrap and refrigerate until the dough is firm, about two hours.
2. Position two racks in the oven so that they are evenly spaced and preheat the oven to 350 degrees Fahrenheit. Line two cookie sheets with Gefen Parchment Paper.
3. Moisten your hands with water, scoop up a tablespoonful of the dough, and roll the dough between your palms into a ball. Place the ball on a prepared cookie sheet. Repeat with the rest of the dough, evenly spacing the balls on the cookie sheets and flattening them a little. You should have 20 balls.
4. Bake the cookies until puffed and slightly firm, eight to ten minutes, rotating the pans halfway through. Let the cookies cool for five minutes, then use a metal spatula to move them directly to wire racks. Let cool.