Preheat oven to 325°F (163°C). Line a baking sheet with parchment paper.
In a medium bowl, combine the melted butter, brown sugar, and granulated sugar. Stir until fully combined.
Add the mashed banana and vanilla extract, mixing until fully incorporated.
In a separate bowl, whisk together flour, cornstarch, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
Gently fold in the chocolate chips.
Using a 2-tablespoon scoop, drop dough onto the lined baking sheet, spacing each about 2 inches apart.
Bake for 12-15 minutes, or until the edges are golden brown. The centers may appear soft but will set as they cool.
Let cookies cool on the baking sheet for 5-10 minutes before transferring to a wire rack to cool completely.