Whisk together flour, cornstarch, sugar, baking powder, salt, and nutmeg in a mixing bowl.
In a small bowl, whisk together the buttermilk, egg, melted butter, and vanilla. Add to the dry mixture and stir till just combined. Gently fold in the sprinkles.
Spoon batter into a ziplock bag and snip off the corner. Squeeze the batter into a well greased donut pan. Divide the batter evenly into the six donut wells so the donuts will bake evenly.
Bake at 375° for 11-12 minutes. Let cool for a couple minutes, then remove to cooling racks. Let cool for a few minutes.
For glaze: Whisk together powdered sugar, butter, vanilla, salt, and milk in a small bowl, adding enough milk to get a thin consistency.
Dip donuts into glaze, trying to cover each donut completely. Place donuts back on cooling rack and add additional sprinkles.