Prepare the banana-infused milk: In a medium saucepan, combine the whole milk, double cream, and 50g of the sugar. Peel one of the bananas, slice it into thin rounds, and add it to the pan. Gently heat over a medium flame, stirring occasionally, until small bubbles appear around the edge — you should see wisps of steam rising, but the mixture should not boil. Remove from the heat, cover, and let the banana steep for 30 minutes. You'll smell a sweet, fruity aroma as the banana releases its flavour.

Strain and reheat: Pour the infused milk through a fine-mesh sieve into a clean jug, pressing gently on the banana solids to extract every last drop of flavour. Discard the solids. Return the strained liquid to the saucepan and heat again over a medium flame until it's hot to the touch — dip a clean finger in quickly; it should feel warm, not scalding.

Temper the egg yolks: In a heatproof bowl, whisk the egg yolks, remaining 100g sugar, cornflour, and salt together until the mixture is pale and thick — it should fall from the whisk in a thick ribbon that holds its shape for a second before disappearing. While whisking constantly, slowly pour about 120ml of the hot milk mixture into the yolk mixture. This raises the temperature of the eggs gently so they don't scramble. The mixture will look smooth and glossy.

Cook the custard: Pour the tempered egg mixture back into the saucepan with the remaining hot milk. Place over a medium heat and whisk continuously. Within 2 to 3 minutes, you'll feel the mixture begin to thicken — the whisk will leave distinct tracks on the bottom of the pan. When it reaches a nappe consistency (coating the back of a spoon and holding a line when you run your finger through it), remove it from the heat immediately. The custard should be smooth, pale yellow, and smell richly of vanilla and banana. Stir in the vanilla bean paste.

Cool the custard: Pour the custard into a shallow dish and press a piece of cling film directly onto the surface — this prevents a skin from forming. Let it cool at room temperature for 20 minutes, then refrigerate for at least 1 hour until it's completely cold and set. The texture should be firm enough to hold its shape when spooned but still silky on the tongue.

Whip the cream topping: In a large, chilled bowl, combine the 300ml double cream, icing sugar, and vanilla extract. Whisk with an electric mixer on medium speed until soft peaks form — when you lift the whisk, the cream should form a gentle peak that flops over slightly. Do not over-whip, or it will become grainy and separate.

Assemble the layers: Select a clear glass serving dish — a 2-litre trifle bowl or a deep rectangular dish works beautifully. Spread a thin layer of the cooled custard on the bottom. Arrange a single layer of vanilla wafers over the custard, overlapping them slightly. Slice the remaining 4 bananas into 1cm-thick rounds and arrange half of them over the wafers. Spread half of the remaining custard over the bananas. Repeat the layers: wafers, bananas, custard. Finish with a generous, even layer of the whipped cream, spreading it right to the edges of the dish. For a professional finish, use a piping bag fitted with a star nozzle to pipe rosettes of cream around the border.

Chill and serve: Cover the dish gently with cling film and refrigerate for at least 4 hours, or preferably overnight. The wafers will soften into a tender, cake-like layer. Just before serving, crush a few extra wafers and sprinkle them over the top, along with a few thin slices of banana brushed with a little lemon juice to prevent browning. The contrast of the crunchy crumb against the soft cream is the final touch that makes this feel like a dessert from a fine-dining menu.