Preheat oven to 350F, and grease a cast iron pan or an 8-inch baking dish, and set aside.
In a large bowl, mix together the mashed banana, milk, honey, egg and vanilla extract until fully combined.
Then to that bowl add the oats, baking powder, cinnamon and salt, and mix until everything is well incorporated.
Gently fold in the blueberries, and pour into the prepared baking dish, and smooth out into an even layer. Then take the butter and cut it into about 5 chunks and evenly distribute them over the top of the oatmeal mixture.
Bake at 350 for 30 to 40 minutes.
Remove from the oven and allow to cool for about 15 minutes. Scoop out and serve warm, or cover the dish and refridgerate until you are ready to eat it.