Preheat the oven to 350 degrees.
Prepare a 9 inch square pan by lining it with parchment paper. Lightly spray the paper with nonstick cooking spray.
If you are using full graham crackers, crush them into crumbs. You can do this with a food processor. If you don't have one, put the graham crackers into a Ziploc-type bag and whack them with a rolling pin.
Melt the butter. I just pop it into the microwave for a few seconds.
Put the graham cracker crumbs into a bowl and mix in the melted butter.
Pour the graham cracker mixture into the prepared pan.
Press down the crumbs, making sure to make an even layer. I usually use a metal measuring cup to do that.
Bake for 5-7 minutes until set.
Let the graham cracker crust cool completely.
Mix together the peanut butter, confectioner's sugar, sweetened condensed milk, and heavy cream using the paddle attachment of a stand mixer. You can also use a hand mixer or, just do it the old-fashioned way, with a spoon.
Make sure you scrape the mixing bowl to incorporate everything.
Pour the peanut butter mixture onto the cooled crust.
The peanut butter mixture is thick and it is difficult to spread. I mushed it down with a spatula and then with my fingers.
Break the chocolate into small pieces and put it into a heatproof bowl.
Melt the chocolate in the microwave in 30 second increments.
Don't have a microwave? See the tips below for melting instructions.
Pour the melted chocolate over the peanut butter mixture and smooth it out with a spatula.
Finish with a light sprinkle of flaky salt. I use Fleur de Sel - because some people in this house are actually French.
Put this very heavy pan into the fridge and chill until set. You can certainly cover it with foil if you'd like. I left it overnight.
When it is thoroughly chilled, remove the mixture from the pan using the parchment paper overlap.
Cut into bars. I would recommend making them relatively small because these are SUPER rich.