Preheat oven to 160 degree C. Line a 20 cm square pan with parchment paper and grease it with butter.

Take butter in a saucepan, melt it gently till it gets into brown butter. Set aside.

Now take Sugar, cocoa, water, salt, vanilla, and eggs in a bowl and whisk till combined.

Now add in the hot brown butter, scrape all the brown bits. Now mix it well.

Add in flour and mix well. Add in reese peanut butter cup and mix well.

Spoon this into the pan and bake for 25 mins or insert a toothpick to check for doneness.

Let it cool completely. Now put this in fridge for 30 mins before slicing.

Serve.