Make the syrup: Add the margarine to a hot saucepan over medium heat. Stir in the brown sugar, maple syrup, 1 teaspoon vanilla paste, ¼ teaspoon cinnamon, and ⅛ teaspoon cardamom. Cook until medium brown while you make the challah. Remove from the heat if it becomes too dark.
In a large mixing bowl with as flat a bottom as possible, add the eggs, water, vanilla extract, salt, and white pepper; mix thoroughly.
Make the chocolate filling: In a small mixing bowl, mix the chocolate spread, 1 teaspoon vanilla paste, ½ teaspoon cinnamon, and ¼ teaspoon cardamom. Set aside.
Make the French toast: Place the challah slices on a cutting board, flat side down. Using a sharp small knife, cut a slice in the challah’s crusty top, and around the sides, without piercing the other side or cutting through the bottom crust at all. Gently open the bread, butterfly style, making sure it does not break. Take 1½ to 2 teaspoons of dark chocolate mixture and spread generously across one side of the bread, coating it almost to the edge. (Make sure it does not go right to the edge, or over, because it will burn). Fold the two sides back together, so it looks like a sandwich with one connected edge. Place each slice of challah in the egg mixture, one at a time, allowing each side to soak for 2 to 3 minutes until the bread is soft and liquid soaks through to the center. (Use a spatula to pick up the bread, not a fork, so it stays together well).
Heat the oil in a large skillet over high heat until shimmering. Add the soaked challah to the pan, no more than two slices at a time. Cook until light brown on both sides, flipping once. Transfer to a tray and cover with foil to keep warm. (Or keep warm in a 300°F oven for up to 10 minutes). Repeat with remaining slices.
Heat the sauce on high and add the bananas slices, flat side down. Cook until fully heated through, then flip and cook until hot. Dust with the chocolate shavings, if using. Serve immediately on top of the warm French toast.