Place the butter and the sugar in a double boiler set over medium heat so the water is gently boiling, and stir until the butter melts.
Add the eggs one at a time, whisking well after each addition to blend them thoroughly into the butter and sugar.
Add the lemon juice, whisk and cook, stirring gently, until the mixture thickens, about 6 to 7 minutes.
Remove the double boiler from the heat, remove the lemon cream container from the double boiler, and let it cool to room temperature.
Finally, stir in the lemon zest.
If you have made the lemon mixture the night before, transfer it to an airtight container and refrigerate. If you have made this the day you’re planning to serve it, spread the pastry with the lemon mixture and refrigerate for at least 2 and up to 3 hours before serving it.