Make the PEANUT BUTTER FILLING
Mix together peanut butter, powdered sugar, butter and salt with an electric mixer until smooth. It will be thick enough to form a small ball in your hands. If the mixture is too sticky, add a tablespoon of powdered sugar at a time, beating it with the mixture after each addition. It may take 2 or 3 extra tablespoons to form a small ball in your hand.
Melt the CHOCOLATE
Fill a medium saucepan with a couple of inches of water and place on medium heat.
Add the chopped chocolate to a medium-sized heat proof bowl that will fit comfortably inside the saucepan, but not touching the water.
Melt the chocolate in the bowl on medium low heat, stirring occasionally until most of the chocolate is melted. Remove from heat and carefully place the bowl of melted chocolate aside, continuing to stir until all the chocolate bits are melted.
Assemble the PEANUT BUTTER CUPS
Line a standard size 12 cup muffin tin with disposable mini or regualar muffin tin liners. Spoon slightly less than a tablespoon of the melted white chocolate in the bottom of each.
Measure a heaping teaspoon of the peanut butter mixture and flatten it gently with your fingers, then place it on top of the white chocolate in each cup.
Spoon another tablespoon of the melted white chocolate over the top of each cup, making sure to cover the peanut butter layer.
Gently tap the baking tray flat on a flat surface to release any air bubbles.
Sprinkle a teaspoon of the chopped peanuts or sprinkles if using on top of each cup to decorate.
Refrigerate the cups for at least 30 minutes to set the candy, then remove and serve.