Prepare Ingredients: Gather all your ingredients and measure them accurately.

Heat Milk: In a medium saucepan, heat the whole milk over medium heat until it's warm but not boiling. Keep an eye on it to prevent scalding.

Separate Egg Yolks: While the milk is warming, separate the egg yolks from the whites. Place the yolks in a separate bowl.

Combine Sweetener, Cornstarch, and Salt: In the bowl with the egg yolks, whisk together the granulated erythritol (or sugar substitute), cornstarch, and salt until well combined.

Add Lemon Zest: Once the dry ingredients are combined, add the lemon zest to the egg yolk mixture. Whisk again to distribute the zest evenly.

Temper Egg Yolk Mixture: Gradually pour the warm milk into the egg yolk mixture while whisking continuously. This process is called tempering and prevents the eggs from curdling when they come into contact with the hot milk.

Return Mixture to Saucepan: Pour the tempered egg yolk mixture back into the saucepan with the remaining warm milk. Place the saucepan back on the stove over medium heat.

Add Lemon Juice: Stir in the fresh lemon juice to the milk and egg yolk mixture. Continue stirring gently to combine all the ingredients.

Cook Custard: Cook the custard mixture over medium heat, stirring constantly, until it thickens. This should take about 5-7 minutes. Make sure to scrape the bottom and sides of the pan while stirring to prevent lumps.

Remove from Heat and Add Butter: Once the custard has thickened to your desired consistency, remove the saucepan from the heat. Add the unsalted butter to the custard and stir until it's completely melted and incorporated.

Strain and Chill: If desired, strain the custard through a fine-mesh sieve to remove any lumps or bits of zest. Then, transfer the custard to individual serving dishes or a large serving bowl. Cover the custard with plastic wrap, ensuring the wrap touches the surface to prevent a skin from forming. Chill in the refrigerator for at least 2 hours or until completely set.

Serve: Once the custard has chilled and set, it's ready to serve. Garnish with additional lemon zest or whipped cream if desired. Enjoy your delicious sugar-free lemon custard pudding!