Prepare your ingredients: Before you turn on the heat, finely chop both types of chocolate until you have a pile of small, even shards. In a small bowl, whisk together the cocoa powder, caster sugar, cornflour, and salt. This prevents lumps from forming later. You should have a uniform, dark, powdery mixture with no pale streaks of sugar visible.

Start the milk base: Pour the whole milk and double cream into a medium saucepan. Place it over a medium-low heat. Do not rush this stage. You want to warm the liquid gently until you see the very first tiny bubbles appearing around the edge of the pan and a delicate wisp of steam begins to rise. The surface should shiver but not boil.

Whisk in the dry ingredients: Once the milk mixture is steaming, sprinkle over your dry cocoa-sugar mixture while whisking constantly. Whisk with purpose, making sure you reach into the corners of the pan. You should feel the mixture begin to thicken slightly after about 30 seconds, and the aroma of toasted cocoa will fill your kitchen.

Add the chocolate: Tip the chopped dark and milk chocolate into the pan. Reduce the heat to low. Switch from a whisk to a silicone spatula or wooden spoon, and stir gently but continuously. Watch as the solid pieces of chocolate begin to melt, turning into dark ribbons that swirl through the milk. Keep stirring until the liquid is completely smooth, glossy, and a deep, uniform brown, with no flecks of unmelted chocolate.

Simmer and thicken: Increase the heat to medium-low and bring the mixture to a very gentle simmer. You are looking for a lazy bubble, not a furious boil. Let it cook for 2 to 3 minutes, stirring often, until it has thickened enough to coat the back of your spoon. When you run your finger through the coating on the spoon, the line should hold clearly.

Finish and serve: Remove the pan from the hob. Stir in the vanilla extract. The smell at this point is absolutely intoxicating. Pour the hot chocolate into your favourite mugs. If you are adding toppings, now is the time. A cloud of softly whipped cream, a few chocolate shavings, or a light dusting of cocoa powder all work beautifully. Serve immediately and take a slow, deliberate sip.