In a large bowl, combine melted and cooled butter with mashed banana and brown sugar. Stir until all items are very well combined. Add vanilla and egg yolk and mix until well combined.
Add flour, then prior to mixing it in, add in cornstarch, cinnamon, baking soda and salt by sprinkling over flour (to best distribute). Once those ingredients are added, mix until just combined. Do not over mix. Fold in chocolate chips, again taking care to not overmix.
Tightly cover bowl, or transfer to a container with a lid, and chill in refrigerator for at least one hour or up to 24 hours.
When ready to bake, preheat oven to 350 degrees F.
Prepare baking sheet by placing a silicone baking mat or parchment paper on it. Scoop dough using a 1oz cookie scoop (approximately 2 tablespoons) onto baking sheet, leaving approximately 2 inches between each ball of dough.
Bake for approximately 12 minutes or until edges are starting to brown, but middle still appears a bit unset.
Remove baking sheet from oven and transfer to a wire cooling rack. Allow cookies to cool on baking sheet for at least ten minutes, then use a spatula to remove them from cookie sheet.