Generously grease a 9×13-inch baking dish with butter or cooking spray. For extra insurance against sticking, consider lining the bottom with parchment paper, leaving some overhang for easy removal.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, salt, and cinnamon if using. Creating this homogeneous dry mixture ensures your casserole will rise evenly and have consistent flavor throughout.
In a separate bowl, whisk together buttermilk, eggs, melted butter, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir just until combined – about 10-12 strokes with a wooden spoon or spatula. You'll still see some small lumps, and that's perfect!
Pour the batter into your prepared baking dish, using a spatula to spread it evenly to the corners. Give the dish a gentle tap on the counter to release any large air bubbles.
In a medium bowl, combine flour, brown sugar, and cinnamon for the streusel. Using your fingers or a pastry cutter, work the cold butter into the dry ingredients until the mixture resembles coarse crumbs with pea-sized butter pieces visible. If using nuts, fold them in gently.
Sprinkle the streusel evenly over the pancake batter. For best results, cover the dish and refrigerate for at least 30 minutes (or overnight if preparing for breakfast the next day).
Preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator while the oven preheats.
Bake uncovered for 35-40 minutes, or until the center is set and a toothpick inserted comes out clean. The top should be golden brown and slightly crisp from the streusel.
Allow the pancake casserole to cool for 5-10 minutes before serving. This brief resting period allows the structure to set, making it easier to cut neat portions.
Serve with optional toppings such as warm maple syrup, fresh berries, whipped cream, or a dusting of powdered sugar.