Preheat oven to 350°F. Line an 8x8 baking pan with parchment, grease with nonstick spray.
Cream butter and brown sugar until light and fluffy, 2 minutes.
Mix in egg, vanilla, and salt until combined.
Gently mix in flour just until incorporated, don't overmix.
Fold in mashed bananas, then fold in 3/4 of the chocolate chips.
Pour into prepared pan, top with remaining chocolate chips.
Bake for 33-37 minutes until edges are golden and center is set.
Cool completely before slicing into 16 squares.