Preheat your oven to 350 degrees
Melt 6 tablespoons of butter in a large bowl
Add 4 ounces of chopped chocolate into the melted butter and whisk until fully melted and smooth
Whisk in an egg yolk and a teaspoon of vanilla extract
Add 2/3 cup of brown sugar and whisk until creamy
Mash a medium overly ripe banana until smooth and add it to the bowl
Add 1 1/4 cups of all purpose flour, 1 teaspoon of baking powder and 1/2 teaspoon of salt and mix until just combined
Fold in 3/4 cup of milk chocolate chips
Use a greased cookie dough scoop to add a heaping tablespoon portions of dough onto nonstick or parchment lined baking sheets
Bake at 350 degrees for 8-10 minutes on a middle rack
As soon as the edges of each cookie look set, but the middle is still a little soft remove the baking sheet from the oven and bang it onto your counter top a few times to slightly deflate the cookies
Let the cookies cool for 5 minutes on the baking sheet before transferring them to a cooling rack
Store in an airtight container at room temperature, in the refrigerator or freezer