In a large mixing bowl, add sugar, cocoa, oil, eggs, and peppermint extract. Whisk until the mixture is smooth and glossy, with no streaks of cocoa remaining.
In a separate medium bowl, whisk together the flour, baking powder, and salt.
Gradually add the flour mixture to the cocoa mixture, stirring with a wooden spoon or rubber spatula until a thick, sticky dough forms.
Cover the bowl tightly with plastic wrap and refrigerate the dough for at least 4 hours, up to overnight.
When ready to bake, preheat the oven to 350°F. Line a few baking sheets with parchment paper.
Add confectioners' sugar to a small bowl and set aside.
Using a 2-tablespoon cookie scoop, portion out the chilled dough (it will still be sticky). Roll each dough ball in the confectioners’ sugar until completely coated.
Arrange the coated dough balls on the prepared baking sheets, spacing them about 2 inches apart.
Bake for 10 to 12 minutes, or until the edges are set. The centers will look slightly underbaked but will firm up as they cool.
Let the cookies cool on the baking sheets for 5 minutes before transferring them to wire racks to cool completely.