Step 1: Prepare the Dairy Base

Pour the milk and cream into a medium saucepan. If using a vanilla bean, split it lengthwise, scrape the seeds, and add both seeds and pod to the dairy. Heat over medium-low heat until small bubbles form around the edges (about 180°F/82°C). Do not allow to boil. If using vanilla extract instead, set it aside for now.

Step 2: Prepare the Egg Mixture

While the milk heats, whisk together egg yolks, sugar, cornstarch, and salt in a medium bowl until the mixture lightens in color and becomes smooth—about 2 minutes of vigorous whisking.

Step 3: Temper the Eggs

This crucial step prevents scrambling. Slowly pour about 1 cup of the hot milk mixture into the egg mixture while continuously whisking. This gradually brings up the temperature of the eggs.

Step 4: Combine and Cook

Pour the tempered egg mixture back into the remaining hot milk in the saucepan, whisking constantly. Cook over medium-low heat, stirring continuously with a wooden spoon or heat-resistant spatula, until the custard thickens enough to coat the back of the spoon (about 4-5 minutes, or 170-175°F/77-80°C).

Step 5: Finish and Strain

Remove from heat. If using vanilla extract, stir it in now. Add the optional tablespoon of butter for extra richness. Pour the custard through a fine-mesh strainer into a clean bowl to remove any lumps or the vanilla bean pod. Place plastic wrap directly on the surface of the custard (prevents skin formation) and cool to room temperature before refrigerating.