In a small bowl whisk together the all-purpose flour, baking powder, and salt. Set aside the flour mixture.
In a large bowl, whisk together the melted butter, granulated white sugar, eggs, sour cream, milk and vanilla extract.
Add the dry ingredients to the wet ingredients and whisk just until combined.
Add in the poppy seeds and lemon zest and gently fold them into the batter with a rubber spatula.
Cover the batter with plastic wrap and let the batter rest at room temperature for 1 hour.
While the batter rests make the streusel by whisking together the flour, sugar and salt. Add in the cold butter and lemon zest and smush everything together with your fingers, or a fork, until large clumps form. Place the streusel in the fridge while waiting for the batter.
Preheat the oven to 350 degrees.
Line 2 muffin pans with 12 liners, lining every other cup so that they are spaced out. After one hour, scoop the muffin batter into the muffin cups. Be careful not to mix the batter around after it has rested, just scoop into each liner. Sprinkle streusel on top of the muffins.
Bake the muffins for 22-25 minutes. Let them cool for 10 minutes in the pan then transfer them to a cooling rack to completely cool.
While the muffins cool make the lemon glaze by whisking the powdered sugar and lemon juice together. Once the muffins are cool, drizzle glaze over each and serve!