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, maple syrup, lemon zest, vanilla, and salt. Whisk and let sit for a few minutes.
Place the bowl onto the saucepan and heat. The bowl should not be touching the water.
Gently whisk for 8-10 minutes or until it begins to thicken.
Add ghee 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.