Whisk the brown sugar and melted coconut oil together in a large bow. Add the ground flaxseed, dairy-free milk, vanilla extract, and peppermint extract. Stir well to combine.

Add the flour, cocoa powder, baking powder, and salt. Stir until combined and dough forms. Let dough chill covered in the fridge for at least 2 hours or overnight.

Preheat the oven to 350F and line two baking sheets with parchment paper. Place the granulated sugar and powdered sugar in two separate small bowls. Use a cookie scoop or tablespoon to scoop balls of dough. Roll in granulated sugar then coat generously in powdered sugar.

Place on prepared baking sheets, leaving room for them to spread. Bake for about 12 minutes, until set. Remove from oven and let cool on the baking sheet for 15 minutes. The cookies will firm up as they cool. Transfer to a wire rack to let cool completely.