1Heat oven to 350°F. Spray cookie sheets with nonstick cooking spray or line with parchment paper. In a large bowl of a food processor or a large bowl, 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. If using food processor, spoon flour mixture into large bowl.
2Add eggs and vanilla; stir by hand just until the dough comes together. The dough will be fairly dry and it will seem at first that there is not enough moisture, but if you keep stirring or use your fingers, eventually the dough will come together.
3Place the 1/4 cup powdered sugar into a shallow dish. Roll dough into 1 1/2-inch balls and roll in powdered sugar to coat. Place them about 2 inches apart on a cookie sheets. Bake for 12 to 14 minutes, until just set around the edges but still soft in the middle. Let set 1 to 2 minutes on cookie sheets. Transfer to cooling rack to cool.