Step 1In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, mix together the oil, cocoa, granulated sugar and peanut butter until combined. Beat in the eggs, one at a time, until combined, then beat in the vanilla.
Step 2In a medium bowl, whisk together the flour, baking powder and salt. Beat the flour mixture into the cocoa mixture until combined. Cover the dough and refrigerate until well-chilled, preferably overnight.
Step 3Heat the oven to 350 degrees, and place the powdered sugar in a bowl. Drop teaspoonfuls of the chilled dough into the powdered sugar, then roll, shaping the dough into a ball. Space the cookies about 2 inches apart on a parchment-lined or greased baking sheet. Bake until the cookies are just set to the touch, 10 to 12 minutes, being careful not to overbake. Rotate the cookies halfway through for even baking.
Step 4Cool the cookies on the baking sheet 5 minutes before transferring to a rack . Sprinkle with additional powdered sugar if desired. Store the cookies in an airtight container; the cookies can also be frozen.