01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease lightly with butter.
02 - In a medium bowl, whisk together flour, sugar, baking powder, baking soda, salt, and lemon zest until well combined.
03 - In a separate large bowl, whisk eggs, melted butter, buttermilk, vanilla, and lemon juice until smooth and blended.
04 - Add dry ingredients to wet ingredients and stir gently until just combined. Be careful not to overmix—the batter should remain slightly lumpy.
05 - Gently fold blueberries into the batter, taking care to avoid crushing them. If using frozen berries, add them directly from the freezer.
06 - Divide batter evenly among the 12 muffin cups, filling each about three-quarters full.
07 - Combine flour, brown sugar, cinnamon, and salt in a small bowl. Cut in cold butter using a fork or your fingertips until mixture forms coarse, crumbly pieces.
08 - Sprinkle streusel mixture evenly over the muffin batter, pressing lightly to adhere.
09 - Bake for 20–22 minutes until tops are golden brown and a toothpick inserted in the center comes out clean.
10 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.