Preheat oven to 350F and spray a 12-cavity donut pan with non-stick baking spray.
In a medium-sized bowl, whisk together flour, sugar, cornstarch, baking powder, baking soda, and salt.
In a separate bowl, whisk together the buttermilk, egg, butter, and vanilla extract until well-combined.
Add the dry ingredients to the wet ingredients, folding in carefully so as not to over-mix. Fold in sprinkles.
Pour the batter into a large ziploc bag and snip off the corner of the bag for a makeshift piping bag.
Pipe the batter into the donut pans, filling each mold 2/3 full.
Bake on 350F for 10 minutes.
Allow donuts to cool for 5-10 minutes in their pan before carefully removing and allowing to cool completely on cooling rack. Allow to cool completely before frosting.
FOR THE GLAZE, whisk together your sugar, vanilla, and 3 Tbsp of heavy cream until well-combined. Add additional heavy cream if needed, stirring in 1 Tbsp at a time.
Add food coloring, if using.
Dunk the bottoms of each donut in the glaze and return to cooling rack (bottom side up, now). Sprinkle immediately with sprinkles.
Allow glaze to set (about 30 minutes) before serving (or, if you're like me, dig right in, just keep some napkins handy!)