For the Doughnuts
Preheat oven to 350°F. Lightly grease doughnut pan; set aside. (Note: If you don’t have a doughnut pan, you can bake these in a 6-cup muffin pan. The shape will just be a little different!)
In a large bowl, add the flour, baking powder, sugar and salt; stir until well combined.
Dice the butter into ¼” cubes. Using a pastry cutter or two table knives, cut the butter into the flour mixture.
Add remaining ingredients (milk, sour cream, vanilla, lemon juice, egg and lemon zest); stir until just combined. (Note: Take care not to overmix batter.)
Using a piping bag or spoon, fill each doughnut cup ¾ full. Bake at 350°F for 9-11 minutes, or until doughnuts are light golden brown and springy to the touch. Let cool before removing from the pan.
For the Glaze
Whisk powdered sugar, milk and lemon juice together in a medium bowl. Once doughnuts have cooled, pour glaze on top and let sit until glaze has hardened. Garnish tops of doughnuts with lemon zest before serving.