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, baking soda, and salt until well combined.
In a separate bowl, whisk together the melted butter, buttermilk, 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; some lumps are okay.
Prepare the streusel topping: In a small bowl, combine 1/2 cup flour, 1/3 cup sugar, and cinnamon. Add the cold butter pieces and 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 in the preheated oven for 18-20 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 tin for 5 minutes, then transfer to a wire rack to cool completely.
Serve warm or at room temperature.