Prep the pan and oven: Heat the oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease lightly.

Make the crumble: In a bowl, mix 1/2 cup all-purpose flour, 1/3 cup light brown sugar, 1/2 teaspoon ground cinnamon, and a pinch of salt.Cut in 4 tablespoons cold unsalted butter until the mixture forms moist, sandy clumps. Chill while you make the batter.

Combine dry ingredients: In a large bowl, whisk 2 cups all-purpose flour, 3/4 cup granulated sugar, 2 teaspoons baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon kosher salt.

Whisk the wet ingredients: In a separate bowl, whisk 1 cup buttermilk, 2 large eggs, 1/2 cup melted unsalted butter (slightly cooled), and 2 teaspoons vanilla extract. Add 1 teaspoon lemon zest if using.

Mix the batter: Pour the wet ingredients into the dry.Stir gently with a spatula just until combined. The batter should be thick and a bit lumpy. Don’t overmix.

Fold in the blueberries: Toss 1 1/2 cups blueberries with 1 tablespoon flour, then fold into the batter with a few gentle strokes.This helps keep them from sinking.

Fill the muffin cups: Divide the batter evenly among the 12 cups, filling them almost to the top for a big dome.

Top with crumble: Sprinkle the chilled crumble evenly over each muffin. Add a pinch of coarse sugar for extra crunch if you like.

Bake: Bake at 400°F for 5 minutes, then reduce the oven to 350°F (175°C) without opening the door. Continue baking 14–17 minutes, until the tops are golden and a toothpick comes out with a few moist crumbs.

Cool: Let the muffins rest in the pan for 5 minutes, then transfer them to a wire rack to cool.Enjoy warm or at room temperature.