Put the cupcake liners in a cupcake pan. They will be about 12 cups depending on how thick you make the layers.
To melt the chocolate, put it along with the butter in a bowl and melt in the microwave for 30 seconds at a time, and take out after each 30 seconds to mix using a fork, until the chocolate is melted. This will keep it from burning.
Pour a melted chocolate layer (about 2 tsp - 0.35 oz - 10 gr) in each cupcake tin (the 12 of them). Put in the freezer to harden the chocolate for about 10 minutes.
For the peanut butter layer:
In a bowl, put the peanut butter, confectioners sugar, butter and salt and melt in the microwave for about 15 to 30 secodns until you get a somewhat liquid mixture. Mix using a fork to combine the ingredients.
Take the cupcake pan out of the freezer and pour a layer of this peanut butter mixture in each cupcake tin (a thicker layer than the chocolate one).
Put the cupcake pan in the freezer again for 5 minutes for the peanut butter mixture to thicken.
Once this time has passed, the the cupcake pan out of the freezer again and add the remaining chocolate to each cup, making sure you cover all 12 ot hem. This is where you can add the corase salt on top if you want to.
Put it all in the freezer once more and let it in the for 30 to 60 minutes until the peanut butter cups are firm.
Once ready, tunr the cups out of the pan and store them in the fridge. They can last up to 5 to 7 days.