1. 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.
2. 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 of the eggs and the melted butter. Add the wet ingredients to the dry ingredients and stir until just combined.
3. Heat a large nonstick skillet or griddle over medium heat. Melt a little of the remaining 1 tablespoon butter in the pan. 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.)
4. Whisk together the half-and-half, vanilla, the remaining 5 eggs, the remaining 3 tablespoons sugar, the cinnamon, nutmeg and 1/4 teaspoon salt in a large bowl.
5. Shingle the pancakes in the prepared baking dish. Pour the egg mixture evenly over the pancakes. Cover the dish with plastic wrap and refrigerate for at least 8 hours up to overnight to allow the pancakes to soak in the custard.
6. Preheat the oven to 180°C. Remove the plastic wrap from the casserole and bake until the custard is set and the center feels firm but springy, 50 to 60 minutes. Let cool 15 minutes before serving.
7. Meanwhile, put the maple syrup and pecans in a small saucepan. Bring to a simmer and cook until the syrup has reduced and thickened slightly, about 5 minutes. Remove from heat and keep warm.
8. Pour the pecan-maple syrup over the casserole. Serve with extra maple syrup on the side.