Prepare the custard base: In a medium saucepan, whisk together the sugar and cornflour. Add the egg yolks and a splash of the milk, whisking until you have a smooth, pale yellow paste with no lumps. Pour in the remaining milk and place the pan over a medium heat. Stir constantly with a wooden spoon — you'll feel the mixture begin to thicken after about 5-6 minutes. Once it releases a gentle puff of steam and coats the back of the spoon with a thick, velvety layer, remove it from the heat. Stir in the vanilla extract. The custard should look glossy and smooth, with a texture similar to thick pouring cream.

Cool the custard: Pour the custard into a shallow bowl and press a sheet of cling film directly onto the surface to prevent a skin from forming. Allow it to cool at room temperature for about 15 minutes — it will feel warm but not hot to the touch. Pop it in the fridge for 30 minutes until it's completely cold and has set to a firmer, spoonable consistency.

Whip the cream: In a large, chilled bowl, pour the double cream and add the icing sugar. Whisk with an electric mixer on medium speed until soft peaks form — the cream should hold its shape when you lift the whisk, but still look billowy and soft. Be careful not to over-whip, or the cream will turn grainy and buttery.

Fold the cream into the custard: Take the chilled custard and give it a quick stir to loosen it. Add a generous spoonful of the whipped cream and fold it in gently with a spatula to lighten the mixture. Then, add the remaining cream and fold until no white streaks remain. The finished mixture should be pale, airy, and smell richly of vanilla.

Layer the pudding: Lightly butter the bottom and sides of your 9x13 inch baking dish. Arrange a single layer of digestive biscuits over the base, breaking a few to fill any gaps. Slice the bananas into rounds about 1cm thick and scatter half of them over the biscuits. Spoon half of the custard-cream mixture over the bananas and spread it into an even layer with the back of a spoon. Repeat the layers: biscuits, bananas, and the remaining custard mixture. The top should be smooth and creamy.

Chill and set: Cover the dish tightly with aluminium foil and place it in the fridge for at least 4 hours, but overnight is far better. As it chills, the biscuits will absorb moisture from the custard and soften into a tender, cake-like texture. The pudding should feel firm and sliceable, with no visible liquid when you tilt the dish.

Serve: Before serving, you can add a final flourish of whipped cream or a dusting of crushed biscuits on top. Scoop or slice the pudding — the layers should hold together neatly, with the bananas still vibrant and the custard silky smooth.