MAKE THE PEANUT BUTTER FILLING
In a medium sized bowl, add the peanut butter and 1/2 cup of the confectioners sugar, as well as the tablespoon of butter and salt, mix with an electric hand blender or stand mixer with paddle attachment until smooth. The mixture should be thick and smooth enough for you to form a small ball in your hands.
If it's too sticky, add the additional two to three tablespoons of powdered sugar, adding one tablespoon at a time, beating with the mixer after each addition. Set aside.
Note: The step of additional powdered sugar is usually needed when your peanut butter is room temperature and runny. This step will totally depend on the texture of your peanut butter. If you can form a small tacky ball without it, go for it!
MELT THE CHOCOLATE
Fill a medium saucepan with a couple of inches of water and place on medium heat.
Add the white chocolate to a medium-sized heat proof bowl that will fit comfortably in the saucepan, but not touching the water.
Let it melt in the bowl on medium low heat, stirring occasionally to combine, 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
Using a one tablespoon measuring spoon, put a scant amount (slightly less than a tablespoon) of the melted white chocolate in the bottom of each of the 12 paper-lined cups.
Measure a heaping teaspoon of the peanut butter mixture and flatten it gently with your fingers, then place it on the layer of white chocolate.
Repeat for all cups.
Spoon another tablespoon of the melted white chocolate over the top of each cup, making sure to cover the peanut butter layer.
You should get about 12 cups from this, but if there is a little left over, improvise to use up the ingredients, like using an extra paper liner on a small plate.
Gently tap the tray flat on the counter 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 tray for at least 30 minutes to set the candy, then remove and serve.