Mash the ripe banana in a large bowl until mostly smooth.
Whisk in eggs, milk, vanilla, cinnamon, brown sugar, salt, and nutmeg until well combined.
Heat a nonstick skillet or griddle over medium heat and melt a small pat of butter.
Pour the custard into a shallow dish. Dip each bread slice for 10–15 seconds per side until coated.
Cook soaked bread 2–3 minutes per side until deep golden and set. Add more butter as needed.
Hold finished toast in a 200°F oven. Top with sliced banana and drizzle with maple syrup to serve.