Preheat the oven to 400 degrees Fahrenheit. Butter or spray the molds in a regular-size muffin pan or fit the molds with paper muffin cups.
In a large bowl, rub the sugar and the lemon zest together with your fingertips until the sugar is moist and the fragrance of the lemon is strong. Whisk in the flour, baking powder, baking soda and salt.
In a small bowl, whisk the sour cream, eggs, vanilla, lemon juice and oil together until well blended.
Pour the liquid ingredients over the dry ingredients and gently stir to blend. Don't overmix. Stir in the poppy seeds. Fill each muffin cup ⅔ full of batter.
Bake for 15 to 18 minutes, or until the tops are golden and a toothpick inserted into the center of the muffins comes out clean. Transfer the pan to a rack and cool for 5 minutes before carefully removing each muffin from its mold. Cool the muffins completely on the rack before icing them.
Make the glaze by placing the powdered sugar in a small bowl and add about 1½ tablespoons of the lemon juice. Stir with a spoon to moisten the sugar, then add enough additional lemon juice, a dribble at a time, to get an icing that is thick, but thin enough to drizzle from the tip of the spoon. Drizzle glaze over the muffins. Serve. Glaze will harden as it rests.