Preheat oven to 375°F. Line a 12-cup muffin pan with paper liners or coat with nonstick spray and set aside.
Melt butter and let cool while you mix the batter.
In a large bowl, mix together flour, sugar, poppy seeds, baking powder, baking soda, and salt.
In another bowl, whisk together yogurt, milk, egg, lemon juice, and lemon zest.
Create a well in the center of the dry ingredients and add wet ingredients. Stir to combine, just until no dry bits remain. If mixture is too thick, add a small amount of milk. Fold in melted butter.
Divide batter equally among muffin cups, filling nearly to the top. Bake 16-20 minutes, until a toothpick inserted into the center comes out clean.