In a medium saucepan over medium heat, combine granulated sugar, zest from the Meyer lemons, and freshly squeezed lemon juice. Whisk until the sugar dissolves, about 2-3 minutes. Do not let it simmer.

In a separate bowl, whisk the egg yolks until smooth. Slowly add about 1/2 cup of the hot lemon mixture to the yolks, stirring constantly to avoid cooking the yolks.

Carefully return the yolk mixture to the saucepan with the remaining lemon mixture. Whisk continuously until blended.

Set the saucepan over low heat and whisk for about 3-5 minutes until it reaches approximately 140°F and thickens.

Increase heat to medium and continue whisking until the mixture thickens further to about 150°F, about 2-5 more minutes.

Remove the saucepan from heat and stir in unsalted butter and salt until smooth.

Allow to cool to room temperature, then transfer to an airtight container. Refrigerate for at least an hour before serving.