Preheat the oven to 375°F and set an oven rack in the middle position. Spray a 12-cup muffin tin with nonstick cooking spray.
In a small bowl, combine the lemon zest, lemon juice, and milk. Let sit until curdled, about 10 minutes.
Meanwhile, in a large bowl, whisk the flour, granulated sugar, poppy seeds, baking powder, baking soda, and salt.
Whisk the egg into the curdled lemon/milk mixture, then add to the dry ingredients along with the melted butter. Whisk until just evenly combined.
Spoon the batter evenly among the muffin cups. Bake until lightly golden brown and a toothpick inserted in the center comes out clean, 20 to 23 minutes. Let the muffins cool in the muffin tin for a few minutes, then turn them out onto a wire rack to cool completely.
Meanwhile, make the glaze: In a small bowl, whisk together the lemon juice and confectioners' sugar. Stir, adding more lemon juice or sugar as needed, until you have a glaze with the consistency of honey. Drizzle the warm muffins with the glaze. Let the glaze set for at least 15 minutes, then serve.