Line a square 8-inch baking dish with wax paper, leaving an overhang.
Place all the ingredients in a medium saucepan. Heat over very low heat, stirring often until melted. Make sure the chocolate doesn’t burn.
Remove the mixture from the heat as soon as the chocolate is melted. Keep stirring until smooth.
Using a rubber spatula, transfer the mixture to the prepared pan, spreading it out evenly and smoothing the top out.
Cover with plastic wrap. Refrigerate until set, about 4 hours.
Uncover and use the wax paper overhang to remove the fudge from the pan and onto a cutting board. With a sharp knife, cut into 16-20 squares.
Keep the fudge squares in the fridge, in an airtight container, separating layers with wax paper.
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Notes
I use natural creamy peanut butter. The only ingredients are peanuts and salt. If your peanut butter doesn't contain salt, add a pinch of salt to the mixture.I highly recommend using a fresh jar when making this recipe. The peanut butter at the bottom of these jars is often quite dry, even if you stirred it thoroughly.The fudge is still a bit soft after the initial four hours. It sets better after an overnight stay in the fridge. It does soften quickly at room temperature, so it's best enjoyed straight out of the fridge.