In a medium mixing bowl, whip the egg yolks and sugar with an electric mixer until light and foamy, about 5 minutes. Add the cornstarch and mix again for 1-2 minutes until smooth and combined.

In a saucepan heat the milk on low heat. Add lemon zest and vanilla bean (or extract). Stir to combine. Now, turn heat to medium and heat milk until a soft boil (little bubbles on the sides and steam starts to form). When the milk is hot, remove from heat. Using a whisk- start adding the beaten egg, sugar, and cornstarch mixture, a little at a time into the hot milk. It’s very important to stir constantly until you achieve a unified mixture.

Put the saucepan back on low heat and continue whisking. Simmer for about 3-4 minutes until the cream begins to thicken. The cream must NEVER boil. In this way, you have a smooth cream without lumps and without chunks of cooked egg.

If you have a cooking thermometer you can check that the temperature never exceeds 85°C (185 F). Otherwise, as soon as you see that bubbles begin to form in the cream, lower the heat. You can even remove it from the heat for a few seconds and put it back. This step is the only difficult step in the Pastry Cream recipe.

Keep cooking on very low heat until the cream is thickened. When the cream is well thickened (it will stick to the back of a wooden spoon and not drip off), immediately pour it into a cold glass container. Important to remove the cream immediately from the hot saucepan to prevent it from cooking longer.

While cooling, cover the cream with plastic wrap. Keep it in contact with the cream. This step is to avoid forming skin on the cream surface. Refrigerate at least 3 hours for it to set.