In a large mixing bowl, mix together cocoa powder, sugar, and vegetable oil. Beat in eggs one at a time; add vanilla.
Combine flour, baking powder, and salt; stir into the cocoa mixture. Cover dough, and refrigerate at least 4 hours.
Preheat oven to 350℉. Line cookie sheets with parchment paper or use silicone baking mats.
Roll dough into 1-inch balls, and then coat with powdered sugar before placing onto prepared cookie sheets.
Bake 10-12 minutes. Let stand on cookie sheet 1 minute before transferring to wire racks to cool.