Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large mixing bowl, whisk together 2 cups all-purpose flour, granulated sugar, baking powder, and salt until well combined.
In a separate medium bowl, whisk together the melted butter, milk, eggs, lemon zest, lemon juice, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined. Do not overmix; the batter should be slightly lumpy.
Prepare the streusel topping by combining 1/2 cup flour, brown sugar, and ground cinnamon in a small bowl.
Add the cold, cubed butter to the streusel mixture. Use your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs.
Divide the muffin batter evenly among the 12 muffin cups, filling each about 3/4 full.
Sprinkle the streusel topping evenly over each muffin.
Bake for 18 to 22 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 tin for 5 minutes before transferring to a wire rack to cool completely.