Preheat the oven to 350. Line 18 muffin cups with paper liners, and set aside.
In a medium bowl, whisk together the flour, sugar, and brown sugar. Grate in the cold butter, then mix together using a pastry blender or two knives, until the pieces of butter are no larger than pea sized. Refrigerate until ready to use.
In a large bowl, rub the lemon zest into the sugar until fragrant. Add the butter and beat until light and fluffy. Add the eggs, vanilla, and lemon juice, and mix until combined. Mix in the flour, salt, and baking powder. Add the sour cream and milk, and mix until just combined.
Scoop the batter into the prepared muffin cups, filling 3/4 full. Sprinkle liberally with the crumb topping. Bake 18-20 minutes, or until the tops spring back when touched lightly and the edges of the muffins are golden brown.