Put the peanut butter, butter and sugar in a food processor. Pulse until it comes together. Add the toffee and pulse again. Remove and, using the palms of your hands, roll mixture into small balls. Place on a tray. Chill for 30 mins.
Melt the chocolate in a heatproof bowl in the microwave on High for 1 min or set over a pan of boiling water (make sure the bowl doesn't touch the water). Using a fork, dip the balls, one at a time, until half covered in chocolate, Roll in the peanuts. Put on a tray lined with baking parchment and leave to harden (about 2 hrs).
Store between layers of baking parchment in a covered container and keep chilled. Will keep for 1 week in an airtight container.