Preheat oven to 350ºF.
Grease and flour a loaf pan and set aside.
Whisk together the flour, sugar, baking soda and salt in a large bowl. Set aside.
In a separate bowl, mix mashed bananas, buttermilk, 2 eggs, 6 tablespoons butter and vanilla until well combined.
Lightly fold banana mixture into dry ingredients with rubber spatula just until combined and batter looks thick and chunky.
Scrape batter into prepared loaf pan. Bake about 55 minutes, or until toothpick inserted comes out clean.
Allow the banana bread to cool fully, then slice into ½-inch thick slices.
In a medium, shallow pan, whisk together the remaining 4 eggs with the whole milk and cinnamon. Set aside.
Heat a large non-stick pan over medium heat. Add 2 to 3 tablespoons of butter to the pan.
Dip one of the slices of banana bread into the egg mixture, coating it on both sides.
Allow the excess liquid to drip off then transfer the coated bread to the hot skillet. Pan-fry the bread for about 3 minutes per side, then repeat the coating and frying process (adding more butter to the pan as needed) with the remaining slices of bread.
Serve the banana bread French toast warm, topped with maple syrup and sliced bananas (optional).