Preheat your oven to 350°F (175°C). Generously spray a mini donut pan with non-stick baking spray.

In a medium bowl, whisk together the flour, cocoa powder, granulated sugar, brown sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together the buttermilk, egg, melted butter, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients. Gently fold with a rubber spatula just until combined. Do not overmix.

Transfer the batter to a piping bag (or a large zip-top bag with the corner cut off). Pipe the batter into the prepared mini donut pan, filling each cavity about 2/3 full.

Bake for 8-10 minutes, or until the donuts spring back when lightly touched.

Let the donuts cool in the pan for 5 minutes before transferring them to a wire rack to cool completely to room temperature.

While cooling, make the glaze: Sift the powdered sugar and cocoa powder into a bowl. Whisk in the vanilla and 2 tablespoons of milk. Add more milk, a half-teaspoon at a time, until the glaze is thick but pourable.

Dip the top of each cooled donut directly into the glaze. Let excess drip off, place back on the wire rack, and immediately top with sprinkles before the glaze sets.