Preheat the oven to 375 degrees F. Line a muffin tin with paper liners.

In a small bowl, whisk to combine the flour, sugar, and cinnamon. Pour in the melted butter and mix with a fork until crumbs form. Chill in the refrigerator.

In a large bowl, whisk to combine the flour, sugar, baking powder, salt, and citrus zest.

In another large bowl, whisk to combine the melted butter, egg, and buttermilk.

Add the wet ingredients to the dry and stir just enough to bring the batter together. Do not overstir.

Fold in the blueberries gently.

Use a large 3-Tbsp cookie scoop to portion the batter evenly into the 12 lined muffin wells.

Remove the crumb topping from the fridge and break up the crumbs, sprinkling it onto each muffin batter scoop.

Bake for 28-30 minutes, until the crumbs are golden brown. Ensure they spring back when pressed gently.

Enjoy warm or let cool for about 2 hours for best flavor.