Prepare the custard base: In a medium bowl, whisk the egg yolks and sugar together vigorously until the mixture turns a pale, creamy yellow and falls from the whisk in a thick ribbon. This should take about 2 minutes. Sift the cornflour over the top and whisk again until no lumps remain. The paste will be thick and smell sweetly of raw sugar.

Heat the milk: Pour the milk into a heavy-based saucepan and place over a medium heat. Warm it until you see tiny bubbles forming around the edge of the pan and a gentle steam rises. Do not let it boil. You should hear a soft hissing sound, not a rapid bubble.

Temper the eggs: Ladle about 100 ml of the hot milk into the egg mixture, whisking constantly. This warms the yolks gently so they don't scramble. The mixture will thin slightly and become glossy.

Combine and cook: Pour the tempered egg mixture back into the saucepan with the remaining milk. Return the pan to a medium-low heat and stir continuously with a wooden spoon, scraping the bottom and corners. After 3–4 minutes, the mixture will begin to thicken. You will feel the spoon meet resistance, and the pudding will start to bubble sluggishly. Cook for 1 full minute after the first large, slow bubble breaks the surface. The pudding should be thick enough to coat the back of the spoon and leave a clean trail when you run your finger through it.

Finish with butter and vanilla: Remove the pan from the heat immediately. Drop in the diced butter and the vanilla bean paste. Stir gently until the butter is completely melted and the vanilla is evenly distributed. The pudding will look glossy and smell intensely of warm vanilla.

Strain for perfection: For a truly restaurant-quality finish, pass the hot pudding through a fine-mesh sieve into a clean bowl. This catches any tiny lumps of cooked egg. The result will be a smooth, velvety mixture.

Chill and set: Divide the pudding among your serving dishes. Press a piece of cling film directly onto the surface of each pudding to prevent a skin from forming. Refrigerate for at least 2 hours, or until completely cold and firm to the touch.