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 flour, granulated sugar, baking powder, and salt.
In a separate bowl, whisk the melted butter, milk, eggs, lemon zest, lemon juice, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined. Do not overmix; the batter will be slightly lumpy.
In a small bowl, prepare the streusel topping by combining 1/2 cup flour, brown sugar, and cinnamon.
Add the cold cubed butter to the streusel mixture. Using a pastry cutter or your fingers, cut the butter into the dry ingredients 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 in the preheated oven 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.