Preheat oven to 350°F (177°C). Spray donut pan with non-stick spray. Set aside.

Whisk the flour, baking powder, baking soda, poppy seeds, and salt together in a medium bowl. Set aside. Whisk the melted butter, egg, sugar, milk, sour cream, vanilla, lemon juice, and zest until completely combined. Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix. The batter will be thick.

Spoon the batter into the donut cavities—I highly recommend using a large zipped-top bag for ease. Cut a corner off the bottom of the bag and pipe the batter into each donut cup, filling 2/3 to 3/4 of the way full.

Bake for 9-10 minutes or until the edges are lightly browned. Allow to cool for about two minutes then transfer to a wire rack set on a large piece of parchment paper. Bake the remaining donut batter (there is usually enough batter for 1-2 more donuts) and once baked, transfer to the wire rack. Allow donuts to cool down until you can handle them.

Whisk all of the glaze ingredients together. I usually use 2 Tablespoons of milk, but for a slightly thicker glaze you can use 1 Tablespoon. Dip each donut into the glaze and place back on the wire rack as the glaze sets. (The parchment paper can catch the glaze dripping down.)

Donuts are best served immediately. Leftovers keep well covered tightly at room temperature or in the refrigerator for 2 days.