Combine the crushed biscuit with the melted butter and press into the tin.
Melt the 300g/11oz of milk chocolate for the cheesecake in a microwavable bowl in the microwave in 30-second bursts (mixing well after each burst, even if it doesn’t look like it needs it). Alternatively, melt the chocolate in a small heatproof bowl set over a pan of simmering water (do not let the bottom of the bowl touch the water, and stir occasionally until the chocolate melts). Set the melted chocolate aside for 5 minutes until it cools to room temperature.
Pour the double cream into a large clean bowl, add the cream cheese and icing sugar and beat until very stiff and thick (use an electric hand whisk if you have one). It’s ready when it’s the consistency of ice cream.
Add the cooled melted chocolate and crushed hazelnut chocolates and beat until well combined. Spoon the cheesecake mixture on top of the biscuit base, smoothing the top, and transfer to the fridge for 4 hours (or longer) to set.
Remove from the fridge, run a knife around the edge of the tin and gently push the cheesecake out (if it’s a loose-bottom type, you can sit it on a glass or can and gently push the sides of the tin downwards to help).
Melt the remaining chocolate (using the method explained above) and drizzle over the cheesecake. Top with the remaining hazelnut chocolates. Add sprinkles if you fancy.