Preheat oven to 350°F (175°C)
In a large bowl, combine the flour, sugar, brown sugar, cocoa, baking powder and salt. Break up any lumps of brown sugar using a spoon or fork. (I've tried this with a food processor and I guess mine isn't big enough because I just end up making a mess.)
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. It'll resemble a thick brownie batter.
(Optional) Cover and let rest in the refrigerator for an hour or two if you have time.
Place a few heaping spoonfuls of icing sugar into a shallow bowl.
Roll dough into 1½" balls (slightly smaller than a ping pong ball) and roll the balls in powdered sugar to coat. Place them about 2" apart on a cookie sheet that has been sprayed with nonstick spray or lined with parchment paper.
Bake for 12–14 minutes, until just set around the edges but still soft in the middle.
Transfer to a wire rack to cool.