Pour peanut butter chips into a bowl.
Microwave heavy cream until just boiling and pour it over the chips.
Cover the bowl with plastic wrap and leave it alone for 5 mins.
Whisk smooth.
Re-cover and chill 45 mins.
Whisk in mascarpone until smooth and pour over the chocolate cake.
Use an offset spatula to smooth this layer.
Chill a few hours.
Run knife around side and remove outer pan ring.
Pipe on the frosting and drizzle with chocolate syrup.