Preheat a large skillet or griddle over medium heat. Set oven to 200°F to keep toast warm.
Whisk eggs, milk, cream, 1 tsp vanilla, cinnamon, granulated sugar, and a pinch of salt until smooth.
Pour the custard into a shallow dish. Add brioche slices and soak 15–20 seconds per side.
Melt 2 tbsp butter on the hot skillet. Cook soaked bread until golden, 2–3 minutes per side.
Transfer cooked toast to a baking sheet and keep warm in the oven while you make the sauce.
Melt 4 tbsp butter in the skillet. Stir in brown sugar, 1/2 tsp cinnamon, nutmeg, and a pinch of salt. Bubble 1 minute.
Add sliced bananas and cook, stirring gently, until softened and glossy, about 1 minute.
Pour in dark rum. Flambé carefully or simmer 30–60 seconds to reduce. Stir in 1 tsp vanilla and pecans.
Plate the French toast. Spoon warm banana-rum sauce over the top and dust with powdered sugar. Serve immediately.