Preheat the oven to 350°F and grease an 8x8-inch baking dish with cooking spray.
Place the diced dates in a small bowl and pour the boiling water over them. Set them aside to soften. Do not drain the dates; you’ll need the soaking liquid later on.
In a separate bowl, stir together the oats, baking powder, and cinnamon. Set aside.
Add the milk, egg, and vanilla to the prepared baking dish and whisk until combined.
Add the mashed bananas, softened dates along with their soaking liquid, and salt to the baking dish, and stir to combine.
Add the oat mixture and stir until evenly combined.
Bake uncovered for 20 minutes, or until the center is firm to the touch.
Remove the baked oatmeal from the oven and let it rest for 5 minutes before cutting it into squares or scooping onto plates.
Serve with Greek yogurt, maple syrup, or sliced bananas, if desired.