In a large bowl, whisk the cocoa powder, oil and granulated sugar until smooth and glossy.
Whisk in the eggs one at a time, then add the vanilla. Whisk for about 1 minute, until thick and shiny.
Sift the flour, baking powder and salt over the bowl. Fold with a spatula until no dry flour remains. The dough will be soft and sticky.
Cover the bowl and refrigerate for at least 2 hours, or up to 3 days, until the dough is firm enough to roll.
Heat the oven to 350°F (175°C) and line two baking sheets with parchment paper.
Scoop the dough by the tablespoon and roll it into balls between your palms.
Roll each ball in the powdered sugar until thickly coated, let it sit for a minute and roll again for a heavy coat.
Place the balls 2 inches apart on the baking sheets. Don’t flatten them.
Bake one sheet at a time for 10 to 12 minutes, until the cookies have puffed and cracked and the edges look set, while the centers still look slightly soft.
Cool on the pan for 5 minutes, then move to a wire rack to cool completely.
