Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly.
In a large bowl, combine 2 cups of flour, 1 cup of sugar, 1 tablespoon of baking powder, and 1/2 teaspoon of salt. Mix well.
In another bowl, whisk together the melted butter, egg, milk, lemon zest, lemon juice, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
In a separate bowl, prepare the crumb topping by mixing 1/2 cup of flour, 1/4 cup of sugar, cubed butter, and cinnamon. Use a fork or your fingers to combine until crumbly.
Divide the muffin batter evenly among the muffin cups, filling each about 2/3 full. Sprinkle the crumb topping generously over each muffin.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
While the muffins are baking, prepare the lemon glaze by whisking together 1 cup of powdered sugar, 2 tablespoons of lemon juice, and 1 teaspoon of lemon zest until smooth.
Once the muffins are done, allow them to cool for a few minutes in the pan before transferring them to a wire rack. Drizzle the lemon glaze over the warm muffins.