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