In a small bowl, whisk together the sweetener, cornstarch, and salt. Add it to a small saucepan and gently whisk in the milk until combined.
Place the saucepan over medium heat and bring it to a boil. Continue to whisk and let it boil for 1-2 minutes, before reducing the heat to low. Gently scoop out half a cup of the mixure and add it to a bowl with the whisked egg yolks. Whisk very well, until combined. Add the egg yolk mixture into the hot mixture in the saucepan. Simmer for another 3 minutes, before removing from the heat.
Gently whisk in the butter and the vanilla extract until combined. Distribute the pudding mixture amongst 8 small ramekins or serving dishes. Refrigerate the pudding for at least an hour, to firm up.
TO STORE: Vanilla pudding should be stored in the refrigerator, covered. It will keep well for up to 2 weeks. TO FREEZE: Place leftover pudding in single serving containers and store in the freezer for up to 6 months.