Heat the oven to 350°F. Line the bottom and 2 opposite sides of an 8x8-inch square baking pan with a sheet of parchment paper long enough to hang over the sides by a few inches to form a sling.
Peel and mash 2 ripe medium bananas in a large bowl until only a few lumps remain (about ¾ cup). Whisk in ½ cup packed brown sugar, ¼ cup whole milk, 2 teaspoons ground cinnamon, 2 teaspoons vanilla extract, and ¾ teaspoon kosher salt until combined.
Stir in 2 cups old-fashioned rolled oats, ½ cup of the mix-ins, and ½ teaspoon baking powder.
Transfer to the baking pan and spread into an even layer. Sprinkle the remaining 2 tablespoons mix-ins over the top.
Bake until the top and edges are golden-brown and the center is set, 30 to 35 minutes.
Let cool in the pan for 10 minutes. Grasping the excess parchment, lift the slab out of the pan and place on a wire rack (keep on the parchment). Let cool completely, about 1 ½ hours.
Cut into 9 pieces.
