1. Combine the milk, heavy cream, and lemon zest in a saucepan and heat over medium-low until bubbles form at the edges. Remove from the heat, cover, and let steep for 20 to 30 minutes.

2. Whisk the egg yolks, sugar, and cornstarch in a medium bowl until smooth and pale.

3. Rewarm the steeped milk mixture until steaming. Slowly pour a small amount into the yolk mixture while whisking constantly to temper the eggs, then whisk in the rest.

4. Pour everything back into the saucepan. Cook over medium heat, whisking constantly, until the cream thickens and begins to bubble. Simmer for about 1 minute to cook out the starchy taste.

5. Remove from the heat and whisk in the lemon juice and butter until smooth.

6. Strain through a fine sieve into a clean bowl. Press plastic wrap directly onto the surface and refrigerate until fully chilled and set, at least 2 hours.

7. Whisk briefly to loosen before using as a filling or serving with fruit.