Preheat oven to 425 degrees,. Spray two 6-cavity donut pans with nonstick spray and set aside.
In a large mixing bowl, whisk together the flour and cornstarch. Whisk in sugar, baking powder, and salt until well combined. Whisk in the buttermilk, eggs, lemon juice, butter, vanilla, and lemon zest until batter is smooth and combined, but don not over-mix. Evenly divide the batter between the prepared donut pans.
Bake for 7-10 minutes until the edges of the donuts begin to turn a light golden brown and the tops of the donuts spring back when lightly touched. Allow donuts to cool for about 5 minutes in pans before placing on a cooling rack.
For the coating, In a bowl have the melted butter and in other bowl whisk together the sugar and lemon zest. Dip one side of each donut into the butter, and the press into the sugar/zest mixture.
Serve warm or at room temperature. ENJOY!