1. Prepare the ingredients:
In a small bowl, whisk the egg yolks, sugar, cornstarch, and salt together until smooth and well combined. Set this mixture aside.

2. Heat the milk:
In a saucepan, heat the milk over medium heat, stirring occasionally. Bring it to a simmer, but be careful not to let it boil over. Once the milk is hot, remove it from the heat.

3. Temper the eggs:
Slowly pour the hot milk into the egg mixture, a little at a time, while whisking constantly to prevent the eggs from scrambling. This process is called "tempering" and ensures the custard remains smooth.

4. Cook the custard:
Once the egg mixture is fully combined with the milk, pour it back into the saucepan. Cook the mixture over medium heat, stirring constantly. Keep stirring for 5–7 minutes, or until the custard thickens enough to coat the back of a spoon. Do not let it boil, as this can cause the custard to curdle.

5. Add vanilla and butter:
Once the custard has thickened, remove it from the heat and stir in the vanilla extract and butter (if using). The butter will give the pudding a silky, rich texture.

6. Strain and cool:
To ensure your custard is completely smooth, you can strain it through a fine-mesh sieve into a clean bowl. Let the pudding cool for a few minutes, then cover with plastic wrap (press the wrap directly onto the surface of the pudding to prevent a skin from forming). Refrigerate for at least 2 hours to let it set.

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