Preheat oven to 425°F. Spray 2 6-donut donut pans with non-stick cooking spray. Set aside.
In a large bowl, whisk together all-purpose flour, granulated sugar, baking powder, and salt.
Add buttermilk, eggs, butter, and vanilla; mix in until just combined.
Fill each donut pan cavity about 2/3 full. Do not overfill.
Bake donuts 7-9 minutes or until the tops spring back when pressed.
Let donuts cool in pan 5 minutes before removing the donuts to a wire rack to cool.
For chocolate glaze, whisk together the sugar and cocoa powder in a medium bowl.
Whisk in the milk and vanilla until the glaze is smooth and well-combined. Add additional milk as needed to make a thick but pourable consistency.
Dip the top of the donuts into the glaze and place on the wire rack to let the glaze set up.
All-purpose flour: It's important to properly measure the flour. Either weigh the ingredients (click on "metric" directly underneath the ingredients) or be sure to sift the flour before lightly spooning into a measuring cup and level.