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1
Combine the milk and vanilla in a heavy saucepan. Bring to a bare simmer over medium heat. Remove from heat as soon as small bubbles appear at the edges. The milk should be steaming and fragrant.
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2
In a bowl, whisk the egg yolks and sugar together until the mixture is pale and slightly thickened. Add the flour, cornstarch, or combination and whisk until fully incorporated with no dry streaks visible.
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3
Add the hot milk in a thin, steady stream to the yolk mixture, whisking constantly throughout. The stream must be slow at first — add roughly one quarter of the milk before increasing the rate. The tempered mixture will be smooth, pale, and liquid.
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4
Return the mixture to the saucepan over medium heat. Whisk constantly, reaching the edges and base of the pan, as the cream heats. This is where the dish lives or dies: the cream must be brought to a full, bubbling boil and held there for 1–2 minutes with unbroken whisking. The cream will thicken dramatically, then loosen slightly — this loosening signals that the starch granules have fully ruptured and gelatinisation is complete. A spoonful dropped back into the pan should hold a ribbon for 5 seconds. The finished cream is smooth, shiny, and has no floury taste.
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5
Remove the pan from the heat. Allow the cream to cool for one minute — it should no longer be at a rolling boil but remain hot. Add the cold butter pieces and whisk until fully incorporated. The cream will turn glossy.
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6
Transfer immediately to a clean bowl. Press a sheet of plastic wrap directly onto the surface of the cream, leaving no air gap. Cool rapidly — set the bowl over an ice bath if available. The surface beneath the wrap should show no skin and the cream no pooling liquid at the base.