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 medium bowl, whisk together buttermilk, melted butter, 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.
To make the streusel topping, combine 1/2 cup sugar, 1/3 cup flour, and cinnamon in a small bowl. Add the cold cubed butter and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.
Spoon the muffin batter evenly into the prepared muffin tin, filling each cup about 3/4 full.
Sprinkle the streusel topping evenly over each muffin.
Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Serve warm or at room temperature.