Preheat the oven to 180°C (Gas Mark 4 / 350°F).
Combine the flour and caster sugar in a large mixing bowl, and add the butter. Use your fingertips to rub in the butter, until the mixture resembles breadcrumbs. Then bring it together to form a dough.
Lightly grease a baking tray measuring approximately 9 x 13 inches, and press the shortbread mixture into the tray. Bake for around 25 - 30 minutes, or until the shortbread is firm and lightly golden. Remove from the oven, and leave to cool in the tray.
To prepare the peanut butter caramel, add all of the caramel ingredients to a saucepan. Bring to a boil, stirring constantly, then turn the heat down, and continue to cook for another 5 - 10 minutes, until the mixture has thickened slightly. Continue to stir constantly to prevent burning.
Spoon the peanut butter caramel over the cooled shortbread, and spread it out to the edges. Leave to cool completely.
Break the chocolate into squares, and melt in the microwave. Stir it thoroughly every 15 seconds or so - don't overcook it. Mine took just 45 seconds, plus a good stir. Spread the melted chocolate over the cooled caramel, and leave to cool again. Cut into squares to serve.