In the bowl of an electric stand mixer, or large glass bowl if using a handheld mixer, add the cocoa powder, sugar, vegetable oil, and vanilla extract then mix on medium speed until well combined. Add one egg then mix on low speed until just combined before adding the next egg and mixing on low speed until just combined. Scrape down the sides of the bowl.
Add the gluten free flour blend, baking powder, and salt to the bowl then mix on low speed until just combined. Cover the dough then refrigerate for at least 4 hours, or overnight.
Preheat the oven to 350 degrees then line a half sheet pan with parchment paper. Pour the powdered sugar into a shallow dish.
Scoop out 1 Tablespoon dough at a time then roll it into a ball, and roll it in powdered sugar. Place the dough balls onto the prepared sheet pan then bake for 8-10 minutes, rotating the baking pan halfway through. Err on the side of slightly under-baking, vs over-baking. Rap the baking sheet from a few inches above the countertop, then let the cookies cool on the pan for 2 minutes before transferring to a cooling rack to cool completely.