Lemon Custard Filling

Step 1
Combine sugar, cornstarch, and salt in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until sugar is melted, 3 to 5 minutes.

Step 2
Whisk water and egg yolks together in a small bowl; pour into sugar the mixture, whisking until fully incorporated. Cook, stirring constantly, until the mixture thickens, 3 to 5 minutes.

Step 3
Remove from the heat and stir in lemon juice and butter until incorporated. Cover with plastic wrap and let sit until completely cooled.