Combine the sugar and flour in a mixing bowl.

In a separate large mixing bowl, lightly beat the eggs.

Gradually add the flour mixture to the beaten eggs, stirring constantly with a spatula to incorporate smoothly without lumping.

Slowly add the room temperature milk while mixing thoroughly after each addition.

Pour the mixture into a heavy-bottomed saucepan, adding the lemon zest.

Cook over medium-low heat, stirring constantly, until the mixture comes to a boil.

Remove the zest once thickening begins.

Once boiling, cook for an additional minute while stirring.

Remove the saucepan from the heat and transfer the mixture to a chilled container.

Stir in the salt and vanilla.

Continue mixing as the temperature decreases to 50°C, using a cooking thermometer for accuracy. If a thermometer is unavailable, mix for 5–6 minutes, or until no longer hot.

Cover and allow to cool completely before use.