Spoon out the dulce de leche all over the mixture and then, using a spoon or fork, submerge some spread while leaving a part on the top. Top the mixture with the sliced banana.
In a bowl mix the milk, eggs, vanilla extract, and maple syrup (if using). Add the 2,5 cups oats, baking powder, salt, and one ripe mashed banana. Mix well and transfer into a slightly buttered baking dish.