In a large heatproof bowl (preferably with a pour spout), whisk the egg yolks and cornstarch together with a fork. It may not look like it will all come together (it will be dry at first), but keep mixing until the mixture is thick and combined. If it’s not coming together at all, add a few drops of the milk you need in step 3 to bring it together. Set aside at room temperature.

Combine the milk and sugar in a medium saucepan over medium heat. Whisk until the sugar has dissolved, then bring to a gentle simmer. Remove from heat. Pour the warm milk and sugar mixture in a slow and steady stream into the egg yolk and cornstarch mixture, whisking the entire time. Keep those egg yolks moving so they don’t scramble. Pour the mixture back into the saucepan through a sieve, to strain out any egg yolk solids that may have formed during tempering.

Return the saucepan to medium heat, and whisk constantly. The mixture will be frothy on the surface at first, and then it will begin to thicken quickly and large bubbles will begin to burst on the surface. This usually takes about 1–2 minutes. Stand back and use caution. Let the bubbles burst for about 20 seconds, or until the mixture has thickened into a custard- or pudding-like consistency. (For a more accurate test, the custard is done when the temperature reaches 185–190°F (85–88°C).) Remove from heat. Whisk in butter, vanilla extract, vanilla bean seeds, and a pinch of salt. Cool for 10 minutes.

Transfer the pastry cream from the saucepan to a heatproof bowl and place a piece of plastic wrap or parchment directly on the surface of the custard, to prevent a skin from forming. Refrigerate for at least 3 hours, and up to 24 hours. Makes 2–2.5 cups, or about 580–610g.