01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease thoroughly with butter.
02 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Cut in cold butter with a fork or fingertips until mixture resembles coarse crumbs. Set aside in refrigerator.
03 - In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt until well blended.
04 - In a separate bowl, whisk together buttermilk, melted butter, eggs, lemon zest, lemon juice, and vanilla extract until fully incorporated.
05 - Pour wet ingredients into dry ingredients. Gently fold with a spatula until just combined. Do not overmix—small lumps are acceptable.
06 - Gently fold blueberries into batter, taking care not to crush them.
07 - Divide batter evenly among prepared muffin cups, filling each about 3/4 full.
08 - Sprinkle prepared streusel topping generously over each muffin, pressing lightly to adhere.
09 - Bake for 20-22 minutes until golden brown and a toothpick inserted into the center comes out clean.
10 - Let muffins cool in pan for 5 minutes, then transfer to a wire rack to cool completely.