Preheat the oven to 375°F (350°F for sea-level). Brush a donut pan (or muffin tin) with neutral oil or cooking spray.
Whisk together the flour, white sugar, brown sugar, cocoa powder, baking powder, baking soda, and salt.
In a large liquid measuring cup (or small bowl), whisk together the milk, white vinegar, vegetable oil, vanilla extract, and the egg.
Add the wet ingredients to the dry ingredients, stirring until just mixed. Spoon the batter into a large piping bag (or a plastic bag with the tip cut off).
Pipe the batter into the donut pan about 1/2 way full (don't go more than halfway, or the batter will rise too much and you won't get a hole in the center). Bake for about 10-12 minutes, until a toothpick comes out clean.
Let the donuts cool in the pan for about 5 minutes, and then let them cool completely on a rack. Dip each cooled donut halfway into the glaze, letting the excess glaze drip off. Decorate as desired, and let the glaze set completely, about 20 minutes. Serve! Store extras in an air-tight container for up to four days.