Brown the butter: In a small saucepan set over medium heat, melt the butter. Stir the butter occasionally until it turns amber in color and develops little brown specks.
Pour the brown butter into a medium sized mixing bowl. You should be left with at least 80g of brown butter (but not more than 85g.) If you have a little less than 80g, you can add in a few grams of water until you reach 80g.
Mix the peanut butter into the hot brown butter until the mixture is smooth, then set the mixture in the freezer for 5 minutes.
In the meantime, chop up your chocolate bar (if using) or break your chocolate feves in half.
Once the brown butter has chilled a little bit, whisk in the granulated and light brown sugar.
Whisk in the egg and vanilla.
Add in the flour, baking soda, baking powder and salt. Use a rubber spatula to fold the mixture together just until a few flour streaks remain.
Add in the chocolate and fold just until no flour streaks remain.
Now the annoying but necessary part, chill your dough! Cover the bowl tightly with a lid or with plastic wrap, and set the dough to chill in the fridge for at least 30 minutes, and up to 2 days.**
Preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or a silicone baking mat.
Marble dollops of marshmallow fluff on top of the cookie dough.
Scoop out heaping 3 tbsp. scoops of dough. Once you have used all of the marshmallow fluff on top, add on some more. Place the cookies spread apart to allow room for spreading.
Top each cookie with additional chocolate if desired, and bake for 12-14 minutes, or until the edges of the cookies are golden brown.
Immediately after taking the cookies out of the oven, you can take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular. Serve and enjoy!