Add water to a medium saucepan and heat over medium heat until it reaches a soft boil. Start with 1/2 cup and add more if needed,
In a medium heat-proof bowl, add the egg yolks, lemon juice, powdered sweetener, lemon zest,, vanilla, and salt. Whisk and let set for a few minutes.
Place bowl onto the saucepan and heat. The bowl should not be touching the water (see post for How-To pictures).
Gently whisk for 8-10 minutes or until it begins to thicken.
Add butter cubes and continue gently whisking for an additional 5-7 minutes. These times may vary depending on the heat, so don't give up, it WILL thicken.
Remove from heat when the mixture lightly coats the back of a spoon.
Refrigerate until cool and it will continue thickening.