Mash bananas with a fork in a medium sized bowl. Add butter, brown sugar, sugar, egg yolk, and vanilla. Stir until combined.
In a separate bowl, whisk together all-purpose flour, bread flour, cornstarch, cinnamon, baking soda, and baking powder. If you are using unsalted butter, add 3/4 tsp of salt.
Add flour mixture to banana mixture in 3 parts. Mix until completely combined.
Add chocolate and butterscotch morsels and mix until combined.
Line 2 baking sheet pans with parchment paper.
Scoop dough and drop on prepared baking sheet about 2" apart. I use a black handled Farberware ice cream scoop. You can use a smaller scoop or a spoon if you would like smaller cookies.
Place pans in the refrigerator for at least 30 minutes.
Once dough is almost chilled, preheat oven to 350 degrees F.
Bake for 12-14 minutes (turn pan half way through for even baking) or until the edges are beginning to brown. Allow to cool for 10 minutes on baking sheet, then transfer to cooling rack to cool completely.