Line a baking sheet with parchment paper. Mix the sugar, water, and corn syrup in a pot and cook until the hard crack stage, where the sugar becomes light amber at 300°F. Mix in one cup of the roasted peanuts and then pour the mixture onto the parchment paper, spreading it. Let it cool to room temperature. In a food processor, grind the remaining peanuts on high speed until a paste forms (about three to five minutes) and then mix in the salt. Break the caramelized peanuts into small chunks (I do this by covering the peanuts with parchment paper and bashing the slab of caramel with a hammer) and add the chunks to the peanut butter in the food processor. Process completely in the food processor until no chunks remain. Add more salt if necessary.
Assemble the peanut butter cups:
Add about a teaspoon of chocolate to the bottom of a cupcake liner. Using a piping bag with a large tip, so that caramel chunks don't get stuck, pipe the peanut butter directly to the bottom of the chocolate. Use your finger to pat down the peanut butter and cover with some more chocolate. Place the cupcake liners on a baking sheet and hit the pan against the counter a few times to level the chocolate and get rid of any bubbles. If you didn't temper your chocolate, you must keep the peanut butter cups in the fridge. Avoid touching them with bare hands since you'll leave fingerprints and people will be suspicious as to who touched their chocolate.
Recipe by IronWhisk at http://www.ironwhisk.com/2012/12/peanut-butter-cups/