Place the cocoa powder, granulated sugar and vegetable oil in a bowl. Whisk until thoroughly combined.
Add the eggs one at a time, whisking well in between each addition.
Stir in the vanilla extract.
Add the flour, baking powder and salt to the bowl. Stir until just combined.
Cover the bowl of dough, then chill for at least 2 hours.
Preheat the oven to 350 degrees F. Line two sheet pans with parchment paper or nonstick baking mats.
Roll the dough into tablespoon sized balls, then roll each ball in powdered sugar. Place the balls of dough 2 inches apart on the sheet pans.
Bake each pan of cookies for 10-12 minutes or until edges are set. Cool for 5 minutes on the pan, then transfer the cookies to a rack to cool completely. Serve, or store in an airtight container at room temperature for up to 4 days.