Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, combine the brownie mix, eggs, vegetable oil, and all-purpose flour. Stir well with a heavy spoon until a thick, sticky dough forms.
Place the powdered sugar into a shallow bowl with high sides.
Scoop out 1.5 tablespoon-sized mounds of dough. Roll briefly between your hands to form into balls, then drop them directly into the powdered sugar.
Roll the dough heavily in the powdered sugar until a thick, stark white coating is achieved. Do not shake off the excess.
Place the coated dough balls 2 inches apart on the prepared baking sheets.
Bake for 9-11 minutes. The cookies will puff and crack, revealing the soft, fudgy center.
Let the cookies cool on the hot baking sheet for 3 minutes before carefully transferring them to a wire rack to cool completely.