1. In a heavy-bottomed saucepan, warm the milk over medium heat until it begins to steam but does not boil. Remove from heat.

2. In a separate bowl, whisk together the egg yolks, sugar, cornstarch, and salt until pale and slightly thickened.

3. Slowly pour 1/2 cup of the warm milk into the egg mixture while whisking constantly to temper the eggs.

4. Gradually whisk in the remaining warm milk, then pour the entire mixture back into the saucepan.

5. Cook over low to medium heat, stirring constantly, until the custard thickens and coats the back of a spoon.

6. Remove from heat and stir in the butter and vanilla extract until smooth and glossy.

7. Strain the custard through a fine-mesh sieve into a clean bowl to ensure smoothness.

8. Press plastic wrap directly onto the surface of the custard and let it cool at room temperature, then refrigerate.

9. Serve chilled or slightly warmed, garnished as desired.