Preheat your oven to 350 degrees Fahrenheit.
In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, whisk together the melted butter, sugar, eggs, sour cream, milk, and vanilla extract.
Add the dry ingredients to the wet ingredients and whisk until just combined.
Gently fold in the poppy seeds and lemon zest.
Cover the batter with plastic wrap and let it rest at room temperature for 1 hour.
While the batter rests, make the streusel by combining the flour, sugar, salt, butter, and lemon zest until clumps form. Place in the fridge.
Line 2 muffin pans with 12 liners, spacing them out. Scoop the muffin batter into the cups, then sprinkle streusel on top.
Bake for 22-25 minutes, then cool in the pan for 10 minutes before transferring to a cooling rack.
While the muffins cool, make the glaze by whisking together the powdered sugar and lemon juice. Drizzle over the cooled muffins and enjoy every tangy, lemony bite!