In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until combined. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, add the vegetable oil, granulated sugar, oat milk, and vanilla extract. Beat on medium-high speed until combined.
Fold in the dry ingredients and beat on medium-low speed until combined.
Cover the dough with plastic wrap and refrigerate for at least 1 hour (or up to 24 hours).
Once you’re ready to bake the cookies, preheat the oven to 350°F (180°C). Line a large cookie sheet with parchment paper. Fill a small bowl with powdered sugar and one with granulated sugar.
Form the dough into 1-inch round balls. Dip each dough ball in the granulated sugar first, then in the powdered sugar until it is completely covered on all sides.
Place the cookies 2 inches apart on the prepared baking sheet.
Bake for 12 minutes in the preheated oven.
Allow cookies to cool for 5 minutes on the baking sheet, then transfer to a cooling rack to cool completely.