In a medium mixing bowl, combine flour, cocoa powder, baking powder, and salt. Set aside.
In a separate bowl, whisk together brown sugar, oil, vanilla, and eggs until fully combined.
Gradually add the dry mixture to wet until fully incorporated. The mixture should resemble a thick brownie batter. Cover, and refrigerate for 30 minutes to 2 hours. Stir in chocolate chips if using.
Preheat oven to 350°F. Line 2 baking sheets with parchment paper.
Combine icing sugar and granulated sugar in a shallow dish.
Roll chilled dough into rounded tablespoon balls, dusting hands with icing sugar as needed. Roll cookies in icing sugar, then place on prepared baking sheets 2 inches apart.
Bake for 9-11 minutes or until edges are firm. Allow to cool for 5 minutes on baking pans before transferring to a wire rack to cool completely.
