Preheat the oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper.
Mash the bananas in a large bowl until smooth.
Add wet ingredients: stir in peanut butter, maple syrup, and vanilla extract until combined.
Combine dry ingredients: in a separate bowl, mix oats, cinnamon, baking powder, salt, and chopped nuts.
Mix together: fold the dry ingredients into the banana mixture until evenly combined.
Spread in the pan: pour the batter into the prepared pan and flatten the top with a spatula.
Bake for 25–30 minutes, until the top is golden and firm to the touch.
Cool and slice: let the bars cool completely before cutting into squares or rectangles.