meanwhile, make the filling
In a medium heatproof bowl, whisk the egg with the granulated sugar and cornstarch. Gradually whisk in the warm milk. Transfer the mixture to the saucepan and cook over moderate heat, whisking vigorously, until very thick, smooth and shiny, 3 to 4 minutes. Scrape the pastry cream into a bowl and let cool slightly. Whisk in the cubed butter until incorporated. Place a sheet of plastic wrap directly onto the surface of the pastry cream and refrigerate for about 2 hours, until chilled.