Prepare a 9×13 baking dish by lining with parchment paper or foil and then spraying with non-stick cooking 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.
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, almost creamy.
Add the water and vanilla. Add the eggs, one at a time, mixing in between each addition.
Add the flour 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 into the prepared pan.
Unwrap 12 (or 11...) large Reeses PB cups and press them into the brownie batter. Swirl the top with a spatula, so that the PB Cups are covered.
Bake at 350 for about 25-30 minutes, or until a toothpick comes out batter-free. The center of the pan should be mostly set.