Make the custard base. In a medium saucepan, whisk together the caster sugar, cornflour, and a pinch of salt. In a separate small bowl, whisk the egg yolks with about 50ml of the milk until smooth and pale yellow. Pour the remaining milk into the saucepan and whisk thoroughly. Place the saucepan over medium heat and stir constantly with a wooden spoon. After about 3 minutes, you will feel the mixture begin to thicken against the spoon — it should look like thin single cream. When small bubbles appear around the edges and you hear a gentle simmering sound, reduce the heat to low.

Temper the eggs. Slowly pour about half a cup of the hot milk mixture into the bowl with the egg yolks, whisking continuously as you pour. This prevents the yolks from scrambling. You will see the mixture become smoother and slightly warmer. Pour the egg mixture back into the saucepan, whisking steadily.

Cook the custard. Return the saucepan to medium-low heat and stir without stopping. After 2 to 3 minutes, the custard will visibly thicken — it should coat the back of your wooden spoon thickly, and when you drag your finger through the coating, the line should hold its shape. The smell will shift from raw milk to a sweet, creamy aroma. Remove from the heat immediately and stir in the vanilla extract and lemon zest if using. The texture should be luscious and smooth, like a thick pouring cream.

Slice the bananas. While the custard cools slightly, peel the bananas and slice them into rounds about 1cm thick. Try to keep the slices even so each spoonful gets a consistent distribution. Set aside five or six of the nicest slices for the top decoration.

Whip the cream. Pour the double cream into a clean mixing bowl. Add the icing sugar. Whisk by hand or with an electric whisk until soft peaks form — the cream should hold its shape but still look silky, not stiff or grainy. You will see the cream catch the whisk marks clearly, and it should feel light and billowy when you lift the whisk.

Layer the pudding. Spread a thin layer of the warm custard (about 3 tablespoons) across the bottom of a 20cm square dish or a deep 1.5-litre serving bowl. Arrange a single layer of vanilla wafers over the custard, breaking some to fill gaps. Scatter half the banana slices over the wafers. Spoon half the remaining custard over the bananas, spreading it gently. Add another layer of wafers, then the remaining bananas, then the final custard layer. The layers should feel sturdy, not swimming in liquid.

Top and chill. Spread the whipped cream over the top layer of custard. Arrange the reserved banana slices and a few extra vanilla wafers on top for decoration. Cover the dish with cling film and refrigerate for at least 2 hours, though 4 hours or overnight is far better. The wafers will soften into a cake-like texture, and the flavours will meld beautifully.