With a hand blender, mix together the cocoa, oil, and sugar. Beat in eggs, one at a time. Add vanilla.
Add in potato starch and baking powder, and mix until the batter resembles thick paste.
Chill batter several hours or overnight covered in fridge. If very liquidy after it's been chilled, you may need to add a bit more potato starch.
Preheat oven to 350F/ 180C .
Roll batter into balls, then roll each cookie ball in the powdered sugar until well coated. Space evenly apart on a lined cookie sheet.
Bake for 10-15 minutes until just starting to crinkle. Let the cookies cool off on the baking sheets for five minutes. Then remove gently with a spatula. They spread out a lot and are delicate to handle.