Preheat oven to 350°F.
In a large bowl or in the bowl of a food processor, combine the flour, sugar, brown sugar, cocoa, baking powder and salt, breaking up any lumps of brown sugar. Add the butter and pulse or stir with a fork, pastry cutter or whisk until the mixture is well combined and crumbly.
Add the eggs and vanilla and stir by hand just until the dough comes together. The dough will be fairly dry – it will seem at first that there isn’t enough moisture, but if you keep stirring, or get in there and use your fingers, eventually it will come together.
Place a few heaping spoonfuls of icing sugar into a shallow dish. Roll dough into 1 1/2” balls and roll the balls in icing sugar to coat. Place them about 2” apart on a cookie sheet that has been sprayed with nonstick spray. Bake for 12–14 minutes, until just set around the edges but still soft in the middle. Transfer to a wire rack to cool.
Makes 20 cookies.