Line an 8x8 or 9x9 inch baking dish with aluminum foil and spray with nonstick spray. Or just spray the pan.
Melt butter in a large saucepan over medium heat. Stir occasionally. The butter will foam up, then back down and start forming brown colored milk solids or “bits”. When it is a nice light brown and it starts to smell like you could drink it straight, that’s when you know it’s done.
(You can skip this step and just melt the butter if you want. The brownies don't taste like brown butter, this step just makes them extra rich.)
While your butter is browning, combine sugar, cocoa, and salt in a bowl or stand mixer.
Once the butter is nice and brown, immediately pour the butter into the bowl with the sugar and cocoa (so that it stops cooking. You don’t want it to burn). Beat the mixture until it is smooth.
Add the water and vanilla. Add the eggs, mixing quickly so they don't curdle (if your butter is still pretty hot).
Add the flour and chocolate chips and mix until just incorporated. Batter will be about the consistency of cement. Have a taste. It’s awesome.
Use a sturdy rubber spatula to spread batter in the prepared pan. Bake for 25-30 minutes until toothpick inserted comes out clean.
Let the brownies cool. Or stick them in the freezer for 10 minutes like I did and call it good.
Peel all the foil off the top of the jar of marshmallow creme. Microwave for 15-20 seconds, or until it just starts to rise above the lip of the jar.
Scoop the creme into a bowl or stand mixer. Add a little peppermint extract and green food coloring. Mix well, scraping sides.
Pour the marshmallow creme over the brownies and spread to the edges.
Top with the chopped thin mints, pressing in lightly with your hands if necessary. Let the marshmallow settle back to room temperature before cutting and serving.