Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving a little overhang for easy lifting. Lightly grease the sides.

Mash the bananas: In a large bowl, mash the bananas with a fork until mostly smooth with a few small lumps.

Whisk in the wet ingredients: Add the egg (or flax egg), nut butter, maple syrup or honey, and vanilla. Whisk until well combined and creamy.

Add the dry ingredients: Stir in the oats, cinnamon, baking powder, and salt until everything is evenly coated. The mixture will be thick and slightly sticky.

Fold in blueberries: Gently fold in the blueberries and any optional add-ins. If using frozen berries, work quickly to avoid streaking.

Spread and level: Transfer the batter to the prepared pan. Use a spatula to press it firmly and evenly into the corners for uniform baking.

Bake: Bake for 22–28 minutes, until the top is set and lightly golden around the edges. A toothpick should come out mostly clean.

Cool completely: Let the pan cool on a rack for at least 30 minutes. This helps the bars set so they slice cleanly.

Slice and enjoy: Use the parchment to lift the slab out, then cut into 9–12 bars. Serve slightly warm or at room temperature.