Adjust oven racks to center position and preheat oven to 425°F. Spray two 6-cavity donut pans with nonstick cooking spray and set aside.
In a large bowl, whisk together flour and cornstarch. Add sugar, baking powder, and salt, whisking until well blended.
Combine buttermilk, eggs, melted butter, lemon juice, vanilla, and lemon zest. Stir into dry ingredients until the batter is smooth and combined, but do not over-mix.
Evenly divide the batter between the prepared donut pans.
Bake for 7 to 9 minutes until the edges begin to turn light golden brown and the tops spring back when touched. Let cool for 5 minutes in pans before transferring to a cooling rack.
For the coating, pulse sugar and lemon zest in a mini food processor until finely ground. Dip one side of each donut in melted butter, then press into the sugar/zest mixture. Serve warm or at room temperature.