For the cheesecake:
Position a rack in the middle of the oven and preheat the oven to 170C/Gas 3. Line a 20 by 20cm baking tin with foil, letting long flaps overlap on each side. Spray the foil with nonstick spray.
Break up the brown sugar to remove all lumps. In a standing mixer with the paddle attachment, beat the cream cheese and sugar on a medium speed until very smooth. Add the cream and beat slowly; then add the yolks and vanilla, mixing until just combined. Gently stir in peanut butter. Take care not to over beat the mixture; this incorporates too much air and causes the cheesecake to crack.
Pour the batter over the prepared baking tin, making sure it is evenly spread. Bake for 15 minutes. Open the oven to release some heat, then lower the temperature to 93°C.
Continue to bake the cheesecake until the outside is set but the centre is still loose, for about 45 minutes. Turn off the oven, and cool cheesecake in the oven for 45 more minutes.
Remove the cheesecake from the oven, cover and refrigerate at least eight hours or overnight. Lift cheesecake from the pan by lifting up the foil. Transfer to a cutting board.
Cut the cheesecake into 3 1/2cm cubes. Stick a wooden cocktail stick halfway into each cube and freeze for one hour.
For the chocolate:
Meanwhile, put the shortening and chocolate in a medium heatproof bowl. Bring a saucepan filled with 2.5cm or so of water to a very slow simmer and set the bowl over, but not touching, the water.
Stir the chocolate occasionally until melted and smooth. Remove from the heat and allow to cool slightly. (Alternatively, put the chocolate in a medium microwave-safe bowl. Melt at 50% power in the microwave until soft, about one minute. Stir, and continue to heat until completely melted, about one minute more.)
Dip each cube of cheesecake into the chocolate mixture, and stand on waxed paper to set, about five minutes. Serve cold (straight from the fridge), or freeze up to two weeks. Frozen pops make a cool summer treat, or can be allowed to temper for ten minutes at room temperature before serving.
Cook's Note: Coat the pops with chopped nuts or candies for a really decadent finish. To do this let the dipped pops drip over a bowl for 15 seconds before rolling them.