2. In a large bowl, rub the sugar and lemon zest together with your fingertips until the sugar is moist and the fragrance of lemon strong. Whisk in the flour, baking powder, baking soda and salt. In a large glass measuring cup or another bowl, whisk the sour cream, eggs, vanilla, lemon juice and melted butter together until well blended. Pour the liquid ingredients over the dry ingredients and, with the whisk or a rubber spatula, gently but quickly stir to blend. Don't worry about being thorough - a few lumps are better than over mixing the batter. Stir in the poppy seeds. Divide the batter evenly among the muffins cups.