There is an option to chill cookie dough for 1 hour before baking, This way cookies do not spread. If you prefer not to chill dough, preheat oven to 350 F (175 C) before you start mixing the dough. Line a cookie sheets with parchment paper or Silpat.
In a large bowl sift flour, add salt, cornstarch, baking soda and baking powder. Stir.
In the bowl of a stand mixer cream butter and brown sugar for 2 minutes until fluffy. Make sure you scrape sides of the bowl.
Add peanut butter, egg, banana and vanilla extract and beat on medium speed until well combined, scraping the sides of the bowl if needed.
Add in dry ingredients and mix until just combined.
Using a small cookie scoop or a 1 tbsp measure dough and form balls. Line on baking sheets and press with fork, to make criss cross pattern (optional).
Chill cookies for 1 hour before baking (optional). Bake at 350 F for 8 minutes. Take out of the oven, cool on baking sheet for at least 5 minutes and transfer to a baking sheet.
Meanwhile melt dark chocolate over a double boiler. Fill a zip lock bag and cut the end. Drizzle chocolate over cookies.