Preheat the oven to 375 degrees, and line two baking pans with parchment paper.
In a large bowl, cream together the butter, brown sugar, and granulated sugar until well combined. Add the mashed bananas and the vanilla.
Combine the flour, cornstarch, baking powder, baking soda, and slat in a medium bowl.
Add the dry ingredients into the wet ingredients, and mix until just combined. Then add in the chocolate chips.
Drop tablespoon sized portions of dough onto the parchment paper. The dough will be sticky.
Bake for 9-10 minutes or until lightly golden brown at the edges. Eat warm.