Preheat oven to 350°F. Line a baking sheet with parchment paper.
Make the wet mixture: In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth and glossy.
Add the egg and whisk well until fully combined. Stir in mashed banana and vanilla extract until smooth. After the addition of the banana, the batter will not look so smooth- that is normal.
Add dry ingredients: Sift the flour, baking powder, salt, and cinnamon (if using) directly over the wet mixture.
Use a spatula to gently mix until just combined. Do not overmix.
Fold in the chocolate chunks.
Optional: Cover and chill the dough for 30-60 minutes for thicker-style cookies.
Scoop about 2 tablespoons of dough per cookie onto the prepared baking sheet. Add extra chocolate on top.
Bake for 10–13 minutes, until edges are set, slightly browned, and centers are soft.
Let cool on the pan for 5 minutes, then transfer to a wire rack.
Notes
Use very ripe bananas, they should be spotted and soft for the best flavor
Don’t overmix once the flour goes in, this keeps the cookies tender
Chill the dough if you want thicker, more bakery-style cookies
Slightly underbake, they finish setting as they cool