Assemble the ingredients. Bring the butter to room temperature.
Put the sugar and lemon zest in a bowl and rub together with your hands to mix.
Put the egg, lemon juice, and sugar in a heatproof bowl and mix. Put on a double boiler and continue mixing with a whisk.
When it becomes just a bit softer than custard (about 83℃), remove from the heat and continue mixing.
Keep mixing until it reaches about 60℃ (140℉), then add the butter one tablespoon at a time. Mix with an electric mixer as you go.
Keep mixing for about 5 more minutes, then you're done. Transfer to an airtight container, wrap with plastic wrap, and let cool. When cooled, store in the refrigerator overnight.
If using for a macaron filling, add the almond flour the next day, then fill the macarons.
Use it to fill lemon-flavored macarons!
Here's a recipe for Lemon Macarons. Recipe ID: 1866770.
Story Behind this Recipe
Using the lemon macaron filling recipe from Pierre Herme's book "Macaron" as reference, I came up with a sweet and sour version using less sugar and lemon juice. Mr. Herme's "Macaron" book is an indispensable macaron bible!
Use the freshest eggs you can find. While using the double boiler, be absolutely sure to keep mixing with the whisk. (Be careful not to let the egg turn into scrambled eggs.) It will take a while for it to get to a loose custard-like consistency. So, slowly mix well over low heat.