Preheat oven to 350 degrees Fahrenheit. Spray a 6-count donut pan with non-stick cooking spray and set aside.
In a large bowl, whisk together the flour, baking powder, salt, and poppy seeds.
In a separate bowl, whisk together the granulated sugar, egg, milk, butter, lemon juice, lemon zest, and vanilla until well combined.
Add the wet ingredients to the dry ingredients and mix until just combined, making sure not to over-mix the batter.
Evenly distribute the batter between all of the donut cups. Bake at 350 degrees Fahrenheit for 9 to 10 minutes.
Remove the donuts from the oven and set aside to cool. Meanwhile, in a small bowl whisk together the confectioners sugar and lemon juice. If needed, add a bit more lemon juice to thin the glaze.
Remove the donuts from the donut pan and dip the tops of each donut in the glaze. Transfer each donut to a wire rack with a baking pan underneath to catch any glaze that may fall off. If you have any glaze left over, you can dunk the donuts into it again.