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