Preheat your oven to 350°F (175°C). Grease the mini donut pan with cooking spray.

In a mixing bowl, whisk together the dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In another bowl, mix the wet ingredients: egg, milk, vegetable oil, and vanilla extract until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.

Spoon the batter into the mini donut pan, filling each cavity about 2/3 full.

Bake in the preheated oven for 8-10 minutes, or until a toothpick inserted into the center comes out clean.

Allow the donuts to cool in the pan for 5 minutes before transferring them to a cooling rack.

In a small bowl, whisk together the powdered sugar, cocoa powder, milk, and vanilla extract until smooth.

Dip the cooled donuts into the glaze, allowing the excess to drip off.

Place the glazed donuts back on the cooling rack to set.