In a large bowl, combine cocoa powder with sugar, eggs, oil, and vanilla. Whisk until well combined.
Add the flour, baking powder, and salt, and whisk again until just combined.
Cover with plastic wrap, and chill in the fridge for at least 2 hours (can do up to 48 hours).
Preheat oven to 350°F/180°C, and line a cookie sheet with parchment paper (I use silicone mats).
Remove cookie dough from the fridge, divide the cookie dough into 1 tablespoon balls for a small cookie (20 grams each) or 2 tablespoons for a large cookie (40 grams each). Roll into balls, then roll in powdered sugar and place on cookie sheet about 2 inches apart.
One sheet at a time, bake in a preheated oven for 12-14 minutes, removing from the oven while the centers of the cookies are just a bit underdone.
Allow the cookies to cool on the tray for 10 minutes, then transfer to a wire rack to cool completely.