Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting.
Lightly grease the sides.
Mix the wet ingredients: In a large bowl, mash 2 large ripe bananas until smooth. Whisk in 1/2 cup unsweetened applesauce, 1 egg (or flax egg), 1/3 cup milk, 2 tablespoons maple syrup, 2 tablespoons nut or seed butter, and 1 teaspoon vanilla.
Add the dry ingredients: Stir in 2 cups rolled oats, 1 teaspoon baking powder, 1 teaspoon cinnamon, and a pinch of salt until evenly combined. The batter will be thick and scoopable.
Fold in extras: If using, gently fold in 1–2 tablespoons ground flax or chia, 1/2 cup finely chopped berries, or 1/4 cup mini chocolate chips for older kids.
Spread and level: Scoop the mixture into the pan.
Use a spatula to press it into an even layer, making sure the corners are filled. A flat surface helps even baking.
Bake: Bake for 18–24 minutes, until the edges are set and lightly golden and the center is just firm to the touch. Avoid overbaking to keep them soft.
Cool completely: Let the pan cool on a rack for at least 20–30 minutes.
Lift the slab out with the parchment and cool another 10 minutes before slicing into bars or small squares.
Serve: Offer plain or with a thin smear of yogurt or nut/seed butter. Cut bite-size pieces for younger toddlers.