Preheat the oven to 350ºF. Lightly coat a donut pan with cooking spray.

In a small bowl, whisk together the milk and sour cream. Set aside. In another small bowl, whisk together the flour, baking powder, and salt. Set aside.

In the bowl of a stand mixer, cream together the butter and sugar until light and fluffy. Add egg, lemon zest, and poppy seeds. Beat for 30 seconds, or until combined.

Add half of the flour mixture and half of the milk mixture. Stir gently to combine. Add the remaining flour and milk; stir until just combined.

Transfer the batter into a pastry bag fitted with a large tip (I used a batter dispenser, which was perfect!). Fill each indention in the donut pan about 1/2 full. Bake until the donuts are slightly browned and spring back when touched, about 15 minutes. Remove from the oven and cool completely on a rack. Gently remove from the pan.

In a medium bowl, whisk together the lemon juice and powdered sugar until smooth. Dip the top of each donut into the glaze and dry on a baking rack.