Spray a donut pan with non-stick spray and set aside.
FOR THE DONUTS
In a medium size bowl, whisk together flour, cornstarch, baking powder, salt, cocoa powder, and sugar. Set aside.
In a smal bowl, whisk together egg, butter, buttermilk, and red food coloring until smooth. Pour wet ingredients into dry ingredients and blend until just combined, being careful not to over mix.
In a small prep bowl or a measuring cup (at least 1/4 cup), combine vinegar and baking soda. Mixture will bubble quite a bit. Add vanilla extract and stir to combine. Add this mixture to your batter and stir until everything is incorporated, being careful, again, not to over mix.
Transfer batter to a large zip top bag, snip the corner off, and pipe batter evenly into donut wells, filling them ⅔ to ¾ full. You may also use a spoon.
Bake for 9 minutes or until donuts spring back when you touch them. Cool completely on a wire rack before dipping in glaze.
FOR THE CREAM CHEESE GLAZE
Combine ingredients in a bowl that will accommodate dunking donuts and whisk until smooth. Add more buttermilk 1 teaspoon at a time if you would like it runnier. Add more powdered sugar to firm it up. Dunk donuts into glaze one by one. Transfer to wire rack over a baking sheet or wax paper to catch glaze drippings. If you prefer a crackly plain vanilla glaze, use this recipe.
Dunk donuts as many times as you wish until glaze is gone. Decorate with sprinkles, leave plain, or dust with powdered sugar. Donuts taste the freshest the same day they are baked, but can be stored covered at room temperature up to 2 days.
You may use 1 full cup of all-purpose flour. Please note your donuts will not be as "cakey" as they would if you cut it with cornstarch. Alternatively, you may use 1 cup of cake flour and forego the cornstarch as well.
NO SUBSTITUTIONS. Buttermilk is essential for authentic red velvet flavor.