In a large mixing bowl, beat the butter, brown sugar, and sugar until light and fluffy. Beat in mashed bananas and vanilla.
Whisk together flour, cornstarch, baking soda, baking powder, and salt in a small mixing bowl. Add to the wet ingredients and stir till just combined. Fold in the chocolate chips.
Drop by spoonfuls onto silpat lined or lightly greased cookie sheets.
Bake at 375° for 9-10 minutes or till lightly golden brown around the edges. Cool for 3-5 minutes on the cookie sheet, then remove to cooling racks.