Preheat oven to 400°F. In a large bowl, combine flour, baking powder, sugar and salt. Add sour cream, egg, butter, lemon juice, lemon zest and vanilla extract, stirring until just combined.
Spray donut pan with non-stick spray. Spoon the batter evenly in the donut pan.
Bake for 9-10 minutes or until stick comes clean. These are a sponge-like consistency so don't expect it to be really browned on top. Remove from oven and allow to cool a backing rack.
While donuts are cooling, whisk together the powdered sugar, lemon juice and vanilla extract.
Dip the cooled donuts in the glaze and allow to set for 30 minutes before serving.
Notes
Use fresh lemon juice. Bottled lemon juice isn't the same. Plus you need that zest for the extra kick!
Even though they say non-stick, spray your donut pans with non-stick cooking spray to make sure the donuts come out.
When measuring the flour, lightly spoon it into a measuring cup and then level it off with the back of a knife to get the right amount.
You can spoon the batter into the donut pans, but the easiest way to get the batter in the pan is by using a pastry bag or Ziploc bag with the corner cut off.
Let the donuts cool completely before you dip them in the glaze.
Leftovers will store well in at room temperature for 1-2 days in an air-tight container.