For the Lemon Poppy Seed Muffins

1. In a small bowl, whisk together the all-purpose flour, baking powder, and salt. Set this dry mixture aside for now.
2. In a large bowl, whisk together the melted butter, granulated white sugar, eggs, sour cream, milk, and vanilla extract until smooth.
3. Pour the dry ingredients you set aside into the wet ingredients. Whisk just until everything is combined; do not overmix.
4. Add in the poppy seeds and lemon zest. Use a rubber spatula to gently fold them into the batter.
5. Cover the bowl of batter tightly with plastic wrap. Let the batter rest at room temperature for one full hour before baking.

For the Lemon Streusel

1. While the muffin batter is resting, prepare the streusel topping. In a medium bowl, whisk together the flour, sugar, and salt.
2. Add the cold, cut-up butter and the lemon zest to the dry ingredients. Use your fingers or a fork to work everything together until large, crumbly clumps form.
3. Place the streusel mixture into the refrigerator to chill while you wait for the muffin batter to finish resting.

Baking the Muffins

1. Preheat your oven to 350 degrees F (175 degrees C).
2. Line two standard muffin pans with a total of twelve paper liners. For best results, place liners in every other muffin cup to allow space for spreading. After the one hour rest period, scoop the muffin batter into the prepared muffin cups. Be careful not to stir or mix the batter after it has rested; just scoop it gently into each liner. Sprinkle the streusel topping generously over the top of the scooped batter in each cup. (You might have a small amount of streusel left over).
3. Bake the muffins for 22 to 25 minutes. They should be golden brown and a toothpick inserted into the center should come out clean. Let the muffins cool in their pans for ten minutes. Then, carefully transfer them to a wire rack to cool completely before glazing. (A helpful note: If you choose to bake all twelve muffins in a single standard 12-cup pan without spacing, they may need an extra five minutes or so of baking time).

For the Lemon Glaze

1. While the muffins are cooling on the wire rack, make the glaze. In a small bowl, whisk the powdered sugar and lemon juice together until you have a smooth, pourable glaze.
2. Once the muffins have cooled completely, drizzle the lemon glaze over the top of each muffin. Let the glaze set for a few minutes before serving. Enjoy these delightful treats!