Preheat your oven to 375°F (190°C) and prepare a muffin tin with paper liners.

In a medium bowl, combine flour, sugar, cinnamon, and melted butter for the crumble topping. Mix until it resembles coarse crumbs and chill in the refrigerator.

In a large mixing bowl, whisk flour, sugar, baking powder, salt, and optional citrus zest.

In a separate bowl, combine melted butter, buttermilk, and eggs. Pour this mixture into the dry ingredients and mix gently.

Fold the wet ingredients into the dry ingredients until just blended, keeping the mixture slightly lumpy.

Gently fold in blueberries, being careful not to break them.

Fill each muffin liner about three-quarters full and sprinkle with crumble topping.

Bake for 28 to 30 minutes until golden brown and a toothpick comes out clean.

Cool in the tin for about 5 minutes before serving warm or at room temperature.