Preheat oven to 375 degrees Fahrenheit and spray your donut baking pans that has 6-wells.
In a bowl, add the milk + lemon juice, whisking together well and letting it sit aside for 5-6 minutes until "activated" and the milk thickens a bit. This creates the "buttermilk".
In a separate bowl, add the flour, baking powder, baking soda, sugar, lemon zest, and sea salt. Finally, stir in the buttermilk and melted butter, until combined and batter is thick and smooth, about 1-2 minutes. Fold in the poppy seeds.
Spoon the batter into a piping bag and pipe batter it into prepared donut pan wells, filling them about 3/4 of the way.
Bake for 10-13 minutes or until toothpick inserted into donuts comes out clean.
Remove from oven and allow to cool for 5 minutes before releasing the donuts from their pan and unto a cooling rack to cool completely.