To Prep: Preheat oven to 400°F and line a muffin tin with baking cups.
To Make The Crumb Topping
Add sugar, flour, cold butter cubes and lemon zest to a medium bowl.
Combine using a fork, your fingers, or a pastry cutter, until a crumbly mixture forms. Set aside.
To Make The Muffins
Whisk together the flour, baking powder, salt, and poppy seeds in a large bowl.
In another large bowl, beat together the butter, sugar, lemon zest, vanilla, and lemon extract with an electric mixer, until just combined.
Mix in the egg until combined, then mix in one egg yolk at a time.
Alternate adding the dry ingredients and the milk, mixing as you make each addition, until combined.
Finally, mix in the lemon juice.
Scoop batter into prepared pan so each muffin cup is about 3/4 of the way full. Sprinkle about 2 1/2 TB of the crumble on top of each muffin cup so the batter is completely covered.
Bake at 400°F for 20-25 minutes.
To Make The Lemon Glaze
In a small bowl, whisk together all the glaze ingredients until combined and smooth. If the glaze is too runny, mix in another 2 TB of powdered sugar until you have a consistency that is thick, but pourable.
Scoop into the piping bag with a small tip (or the end cut off of a disposable piping bag).
Drizzle glaze onto cooled muffins and allow to harden before removing from pan, serving, or storing.
Notes
Nutrition information is an estimate only and will vary based on exact ingredients used.