Preheat the oven to 350°F and grease an 8x8-inch baking dish.
In a large bowl, stir together the oats, baking powder, cinnamon, and salt.
In another bowl, whisk the mashed banana, eggs, milk, maple syrup, melted butter, and vanilla.
Pour the wet ingredients into the dry and stir to combine. Fold in 1 cup of the blueberries.
Spread in the baking dish and scatter the remaining blueberries on top.
Bake 32–38 minutes, until golden and set in the center.
Cool at least 15 minutes before cutting into 9 squares.