Prepare the chocolate base: Place the chopped dark chocolate, milk chocolate, cubed butter, and a pinch of salt in a large heatproof bowl. Pour 200ml of the milk into a small saucepan and warm over medium heat until it just begins to steam — you'll see tiny bubbles forming around the edges, and the smell will turn warm and milky. Pour the hot milk over the chocolate mixture and let it sit for 2 minutes without stirring. The chocolate should look glossy and begin to soften around the edges.

Make the chocolate sauce: Whisk the chocolate and milk together until smooth and silky — the mixture will turn a deep, rich brown and smell intensely chocolatey. In a separate small bowl, whisk the cornflour with 2 tablespoons of cold milk until no lumps remain. Pour this slurry into the chocolate mixture, then stir over a low heat for 2–3 minutes until the sauce thickens enough to coat the back of a spoon. It should feel heavy and look like thick pouring custard. Remove from the heat and stir in 1 teaspoon of vanilla extract. Set aside to cool for 10 minutes — it will continue to thicken as it sits.

Whip the cream: Pour the cold double cream into a chilled mixing bowl (a metal bowl straight from the fridge works brilliantly). Add the remaining 1 teaspoon of vanilla extract and the caster sugar. Whisk by hand or with an electric mixer until soft peaks form — when you lift the whisk, the cream should hold a gentle curl that flops over slightly. Be careful not to overwhip; if the cream turns grainy and looks like butter, you've gone too far. Stop at the soft peak stage for the lightest texture.

Layer the biscuits: Break the digestive biscuits into rough chunks — some should be the size of a 50p coin, others smaller crumbs. Spread a third of the broken biscuits across the bottom of a 9x13 inch baking dish or a large glass trifle bowl. The layer should be even but not tightly packed — you want gaps for the chocolate to seep through.

Add the chocolate layer: Pour half of the cooled chocolate sauce over the biscuit layer. Use a spatula to spread it gently, ensuring it drips down between the biscuit pieces. You'll see the chocolate pooling in the gaps while the biscuits stay visible — this creates the lovely marbled effect later.

Add the cream layer: Spoon half of the whipped cream over the chocolate layer and spread it evenly with the back of the spoon. The cream should sit in a smooth, white blanket that contrasts with the dark chocolate beneath. Repeat the layering: remaining biscuits, remaining chocolate sauce, and finally the remaining whipped cream on top.

Finish and chill: Smooth the top layer of cream with a spatula until it's even and slightly swirled. If using fresh berries, scatter them over the top now — they'll sink slightly into the cream as it sets. Cover the dish tightly with aluminium foil and refrigerate for at least 1 hour, though 2–3 hours is better. The finished trifle should feel firm when you gently shake the dish, and the biscuits will have softened into a cake-like texture. Serve chilled.