Melt the butter in a small saucepan on medium.
It will melt, bubble, and then foam. Stir constantly to assess the progress.
After a few minutes, it will turn amber and smell nutty, and brown specks will rise to the top. Your brown butter is done.
In a large bowl mix the hot butter with the sugar for one minute (the mixture will be coarse).
Add the banana and whisk again for 1 minute.
Pour in the egg and vanilla. Mix until creamy.
Fold in the flour, baking soda, and sea salt.
Add 120 grams of the chocolate and mix.
Place the dough in the fridge for at least 1 hour.
Preheat the oven to 170°C (338°F).
Use a medium cookie cutter or spoon to scoop out the cookie dough.
Space them apart on a lined baking sheet.
Bake for around 8 minutes until the edges are golden and the center is just set.
Bang the baking sheet on a hard surface when the cookies are just out of the oven.
Shape the warm cookies with a round glass or large round cookie cutter.
Transfer to a baking sheet and bake the next batch.