This one-pot breakfast casserole is bound to please anyone with a love of sweet-and-savory dishes. Try using cooked bacon or ham instead of sausage, and Swiss or Gouda cheese instead of Cheddar for your own customized casserole.
Butter an 8-inch-square baking dish; set aside. Put 3 tablespoons of the butter in a microwave-safe small bowl or liquid measure; microwave for 30 seconds to 1 minute to melt the butter.
Whisk together the flour, baking powder, baking soda, 3 tablespoons of the sugar and 1/2 teaspoon salt in a large bowl. In a separate small bowl, whisk together the buttermilk, 2 eggs and the melted butter. Add the wet ingredients to the dry ingredients and stir until just combined.
Heat a large nonstick skillet or griddle over medium heat. Melt a little of the remaining 1 tablespoon butter in the skillet. Working in batches, pour the batter by 1/4 cupfuls into the skillet and cook until the bottoms are golden brown and small bubbles appear on the tops, 2 to 3 minutes. Flip and cook until golden brown on the other sides, about 2 minutes. If the pancakes brown too quickly, reduce the heat to medium-low. Transfer the pancakes to a plate as finished. Repeat until all the batter is used, adding more butter to the skillet as necessary. (You should have at least 12 pancakes.)
Whisk together the half-and-half, the remaining 5 eggs, the remaining 1 tablespoon sugar, the pepper and 1/4 teaspoon salt in a large bowl.
Shingle half of the pancakes in the prepared baking dish. Scatter with half of the sausage, sprinkle with half of the cheese and scallions. Pour half the egg mixture evenly over the pancakes. Repeat with the remaining pancakes, sausage, cheese, scallions and egg mixture. Cover the dish with plastic wrap and refrigerate for at least 8 hours or up to overnight to allow the pancakes to soak in the custard.
Preheat the oven to 350ºF. Remove the plastic wrap, cover the casserole loosely with foil and bake for 30 minutes. Uncover and continue to bake until golden brown, the custard is set and the center feels firm but springy, about 30 minutes more. Let cool 15 minutes before serving. Serve warm with maple syrup if you like.