For the caramel: Add the sugar and golden syrup into a small saucepan. Let it simmer over a medium heat for 3 minutes, stirring often until the sugar has dissolved and turned a golden amber colour. Take off the heat and add the butter and cream, whilst stirring continuously. The mixture will bubble up at this point. Once the butter and cream have been fully combined, pour into a heat-safe bowl and set aside to cool to room temperature.
For the caramelised white chocolate: Preheat the oven to 125C/105C Fan/Gas Mark 1/2. Then finely chop chocolate and pour onto a large non-stick (or lined) baking tray. If using callets, there is no need to chop the chocolate. Bake for 10 minutes, then remove from the oven and stir thoroughly until the chocolate is liquid again. Repeat this process 3 times, making sure to stir the chocolate thoroughly before placing back in the oven. Once the chocolate has turned a golden brown colour remove from the oven and leave to cool to room temperature.
For the sponge: Preheat the oven to 180C/160C Fan/Gas Mark 4. In a large bowl, cream the butter and sugar together until light and fluffy. Then add the eggs one at a time, beating between each addition. Then add the flour, baking powder and salt, mixing again until just combined. Finally, add the melted chocolate and milk, mixing again until just combined. The mixture will be thick! Divide the mixture between 12 cupcake cases, and bake for 20 minutes until springy to the touch. Leave them to completely cool.
For the buttercream: In a large bowl, using an electric whisk beat the butter until very light and pale. Then add the icing sugar (covering your bowl with a tea-towel to stop an icing sugar cloud!) and beat until well combined - at least 5 minutes. Finally, add the melted chocolate and milk, mixing until combined.
With the remaining melted chocolate, spread thinly onto a baking tray lined with baking paper. Leave to set in the fridge for 5 minutes. Then mark out circles - I used a small piping tip - then place back in the fridge for 15 minutes. Then snap out the circles you marked, and place back in the fridge until you're ready to assemble the cupcakes.
To assemble: Using an apple corer remove the middles of the cupcakes - you can also do this with a small knife. Then place your room temperature into a piping bag (or just use a spoon) and fill the cupcake holes up to the brim - you won't need all the caramel. Then place the buttercream into a piping bag fitted with a star tip, and pipe swirls on top of each cupcake, covering up the caramel filling. Finally, press a caramelised white chocolate circle into the buttercream of each cupcake.