In a large mixing bowl, combine the flour, sugar, baking powder, cinnamon, and salt. Use a balloon whisk to ensure there are no lumps and that the leavening is well distributed.

In a separate jar or bowl, whisk together the buttermilk, eggs, melted butter, and vanilla extract. If you are feeling adventurous, this is where you add the orange zest.

Create a well in the center of your dry ingredients and pour in the liquid mixture. Use a spatula to fold everything together until just combined. A few streaks of flour are perfectly fine.

Gently fold in half of your blueberries. The remaining berries will be scattered over the top to create those beautiful charred purple craters on the surface.

Preheat your oven to 350 degrees Fahrenheit and brush a 9x13 baking dish with a little extra butter. Pour the batter into your prepared dish. Top with the remaining berries and perhaps a sprinkle of coarse sugar for a crunch. Bake for 35 to 40 minutes. You are looking for a gentle jiggle in the center and edges that have pulled away slightly from the sides of the pan.

Let the casserole sit for five minutes before slicing. This allows the internal steam to settle, ensuring each square holds its shape perfectly.