Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.

In a large mixing bowl, combine the mashed banana, melted butter, and brown sugar. Mix together for about 3 minutes until smooth.

Add the egg and vanilla extract, whisking for about 2 minutes until fully combined.

In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon for about 2 minutes.

Gently fold the dry ingredients into the wet mixture until combined, creating a loose, sticky dough.

Cover the bowl with plastic wrap and chill in the refrigerator for 20-30 minutes.

Scoop 2 tablespoons of dough per cookie onto the prepared sheets, spacing them 2 inches apart.

Bake for 11-13 minutes, until edges are golden and centers are soft.

Let cookies cool on the baking sheet for about 10 minutes before transferring them to a wire rack.