Preheat the oven to 425 degrees F and line a muffin pan with papers.
Place the flour, sugar, baking powder, salt, and lemon zest in a large bowl, and whisk to combine.
Melt the butter in a large liquid measuring cup, then whisk in the sour cream*, milk, eggs, lemon juice, and lemon extract.
Pour the liquid ingredients into the dry, and fold together gently until just barely combined (it's ok if the batter is lumpy).
Divide the batter equally between all 12 wells of the muffin pan, then top with crumb topping.
Bake for 5 minutes at 425 degrees F, then lower the oven temperature down to 350 degrees F and continue to bake for 18 to 22 minutes, or until a toothpick inserted in the thickest part of a muffin comes out clean.
Garnish with powdered sugar, if desired.
To Make the Crumb Topping
Melt the butter in a small pot, then add the flour, sugar, salt, and lemon zest.