Step 1: Preheat and Prepare : Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease lightly with butter.

Step 2: Make the Streusel Topping : In a small bowl, combine brown sugar, flour, and cinnamon. Add the cold butter cubes and mix with your fingertips (or a pastry cutter) until it forms a coarse, crumbly mixture. Chill in the fridge while preparing the batter.

Step 3: Mix the Dry Ingredients : In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.

Step 4: Combine Wet Ingredients : In another bowl, whisk together milk, melted butter, egg, vanilla extract, and lemon zest until well blended.

Step 5: Mix Batter : Pour the wet ingredients into the dry ingredients. Gently stir until just combined — don’t overmix, or your muffins may turn dense. Toss blueberries with 1 tablespoon flour (this prevents them from sinking). Fold the coated blueberries gently into the batter.

Step 6: Fill Muffin Cups : Divide the batter evenly among the muffin cups (about ¾ full). Sprinkle each with a generous spoonful of streusel topping.

Step 7: Bake : Bake for 20–25 minutes, until the tops are golden and a toothpick inserted in the center comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.