Preheat the oven to 350 degrees F. Line a couple baking sheets with parchment paper and lightly grease with cooking spray.
In a large bowl, whisk together the butter, brown sugar, vanilla, baking powder, baking soda and salt until very well-combined.
Add the mashed bananas, egg yolk, and sour cream and stir or whisk until the batter is well mixed.
Add the flour and chocolate chips and mix until no dry streaks remain making sure to scrape the bottom and sides of the bowl well.
Scoop the dough by heaping tablespoons (I use a #40 cookie scoop) and place about 2 inches apart on the prepared baking sheets.
For super puffy cookies, bake without pressing. For slightly less puffy cookies, spray the bottom of a flat-bottomed glass with cooking spray and press in coarse sugar (regular granulated sugar may work, too) and press each cookie gently until slightly flattened, greasing the bottom of the glass as needed to prevent sticking.
Bake the cookies for 10-12 minutes until the cookies are set around the edges and they no longer look wet in the center.