Step 1: Preheat Oven : Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease it.

Step 2: Combine Dry Ingredients : In a large bowl, whisk together flour, poppy seeds, baking powder, baking soda, and salt.

Step 3: Mix Wet Ingredients : In a separate bowl, whisk together melted butter, sugar, eggs, vanilla extract, sour cream, lemon zest, lemon juice, and milk until smooth.

Step 4: Combine Wet and Dry Ingredients : Pour the wet ingredients into the dry ingredients and gently fold until just combined. Do not overmix.

Step 5: Fill Muffin Cups : Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full.

Step 6: Bake Muffins : Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack.

Step 7: Prepare Glaze : In a small bowl, whisk together powdered sugar, lemon juice, and vanilla (if using) until smooth. Adjust consistency by adding more lemon juice for a thinner glaze or more powdered sugar for a thicker glaze.

Step 8: Glaze Muffins : Drizzle glaze over the cooled muffins and allow it to set before serving.