I always start by preheating my oven to 375°F (190°C). While it warms up, grease your baking dish generously with butter. This helps prevent sticking and adds extra flavor to the edges.
In a big mixing bowl, whisk together the flour, baking powder, baking soda, salt, and sugar. This dry mix forms the base of the batter. I always take a moment here to ensure no clumps remain.
In a separate bowl, whisk the eggs, buttermilk, melted butter, and vanilla extract until smooth. The buttermilk and baking soda will give your casserole a nice rise and soft texture.
Now, pour the wet ingredients into the dry. Gently fold with a spatula until just combined. Don’t overmix—lumps are okay! Overmixing makes it dense, not fluffy.
I fold in about 1½ cups of blueberries and reserve the rest. This way, some berries stay whole and juicy inside, while others sit beautifully on top for presentation.
Mix brown sugar and flour in a bowl, then cut in the cold butter until the mixture looks like coarse crumbs. I use my fingers—it’s faster and more fun!
Pour the batter into your prepared dish, smoothing out the top. Sprinkle the remaining blueberries evenly across, followed by the sweet crumble topping.
Bake for 30–35 minutes, or until the top is golden brown and a toothpick comes out clean. I let it cool for 10 minutes before serving—it helps the structure set.