In a large bowl or stand mixer fitted with the paddle attachment, mix together the cocoa, sugar, oil, and orange zest until well combined. Add the egg and vanilla and mix to combine.
In a medium bowl, measure out the oat flour, tapioca flour, 1 teaspoon of the mulling spice blend, baking powder, and salt. Whisk to combine.
Add the flour mixture to the sugar mixture and mix until combined. Cover the bowl with plastic wrap and place in the fridge for 1 hour to firm up.
Preheat the oven to 350° Fahrenheit. Line a baking sheet with parchment paper. Place the powdered sugar and remaining ½ teaspoon of mulling spice in a small bowl.
Once the dough has chilled, scoop out a tablespoon of dough and roll it into a ball. Then roll the dough ball in the powdered sugar, making sure to coat the ball generously. Place the dough ball on the prepared cookie sheet and repeat with remaining dough. Space the cookies at least 2 inches apart.
Bake the cookies in the preheated oven for 12-14 minutes. The cookies should have spread a bit but still have a bit of a rounded top.
Remove the cookies from the oven and allow them to cool on the baking sheet for 5-10 minutes before transferring them to a cooling rack to cool completely. Repeat with any remaining dough.
Cookies can be stored in a lidded container at room temperature for up to 5 days or frozen for up to 6 months.