Place the loaf tin on a flat, heat-resistant surface (a wooden chopping board is ideal). To make the caramel, place a metal pan with the sugar over the lowest heat on your hob and wait for the edges to start melting. At that point, occasionally tilt and shake the pan to melt all the sugar. At no point in the process should you stir the caramel. The caramel is ready as soon as all the sugar is liquid and deep amber in colour. Carefully pour the caramel into the loaf tin and swirl it around to coat the bottom. Leave on the heat-resistant surface to cool.
To make the batter:
Place the shelf in the middle of the oven and preheat it to 180°C (350°F/Gas mark 4).
Place the amaretti in the bowl of a food processor and blitz them at full speed until they turn into a fine, flour-like powder. Set aside. Place the egg yolks and the sugar in the bowl of a stand mixer fitted with the whisk attachment and whisk them at full speed for 2–3 minutes until pale and fluffy.
Meanwhile, place the milk in a spouted, stout, microwave-safe jug and microwave it until it is warm to the touch. Two minutes in a conventional 800W oven should be enough.
When the egg mixture is pale and fluffy, stop the mixer and sift in the cocoa powder, then add the liqueur, vanilla, salt and ground amaretti. Start the mixer at very low speed to combine the ingredients. Finally, add the milk and whisk again until smooth.
Place the loaf tin into the larger roasting tray and pour the batter into the tin. Place the baking tray and tin in the oven and carefully pour boiling water into the tray up to about three quarters of the height of the tin. Bake for 43–45 minutes. At this stage, the surface of the bonèt will look almost spongy and fully baked, with the edges nearly coming off the sides of the tin. The centre will still have a gentle wobble and the batter will feel quite jelly-like. Do not be tempted to wait for the bonèt to look completely set or it will overbake, resulting in a tough, rubbery texture. Remove the loaf tin from the bain-marie and let it cool to room temperature, then leave it to rest in the fridge for at least a couple of hours, ideally overnight.
To assemble:
When you are ready to serve, run a blunt knife around the sides of the baking tin, cover it with an upside-down serving plate and turn both over. Tap the plate over a folded dish towel to help the bonèt come out of the tin. The bonèt should slide out easily, but if it refuses to, dip the tin into a hot water bath for a few seconds, then repeat the previous step.
Decorate the top of the bonèt with a few amaretti biscuits and a few swirls of sweet whipped cream, if you like. Store in the fridge for up to 2–3 days.