Use an enamel, glass, or stainless steel saucepan and combine the lemon juice, butter, and cream. Cook over medium heat just until it scalds, but not boiling.
In a large glass bowl, whisk the eggs, egg yolks and sugar together. Then slowly add ¼ of the hot lemon mixture to the eggs and temper the eggs. Whisk constantly while adding the hot mixture to the eggs making sure that you don't cook the eggs. Continue to add the hot lemon to the egg until it's completely whisked in.
Pour it back into the sauce pan and cook over medium heat. Stir it constantly with a wooden spoon. Scrape along the bottom and sides of the pan watching for the mixture to thicken. To test for proper thickness, remove the spoon and drag your finger across the back of it. You should leave a defined trail behind that does not immediately fill back in.
Strain the mixture through a fine mesh into a clean bowl. Stir in the salt and vanilla.
Seal the bowl or pour it into a mason jar and refrigerate.