Preheat oven to 350F (175C) and lightly grease a 12-count donut pan (I actually use two 6-count) with baking spray (or lightly grease and flour). Set aside.
In a large bowl, whisk together flour, sugars, baking powder, baking soda, cornstarch, salt, and nutmeg (if using).
In a separate bowl whisk together buttermilk, egg, and vanilla extract. While whisking constantly, drizzle in butter until combined (it’s OK if the mixture looks a bit curdled!).
Add the liquid mixture to the dry ingredients and use a spatula to carefully fold together until just combined (all ingredients should be combined, but don’t over-mix or donuts will be dense and dry).
Pipe or spread the batter evenly into prepared donut pans.
Transfer to 350F (175C) oven and bake for 10-12 minutes or a toothpick inserted in the thickest part of a donut comes out clean. Allow to cool for several minutes then carefully remove to a cooling rack.
Use a pastry brush¹ to brush immediately with melted salted butter and then whisk together granulated sugar and cinnamon and dip donuts in mixture. Donuts are best enjoyed warm.
