Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together 2 cups flour, 1 cup sugar, baking powder, and salt until well combined.
In a separate medium bowl, whisk melted butter, milk, eggs, vanilla extract, lemon zest, and lemon juice until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full.
To prepare the crumble topping, combine 1/2 cup flour and 1/3 cup sugar in a small bowl.
Add the cold, cubed butter to the flour mixture. Using your fingers or a pastry cutter, mix until the mixture resembles coarse crumbs.
Sprinkle the crumble evenly over each muffin.
Bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.