In a large metal bowl, using an electric mixer, beat 1 cup of the sugar with the eggs, lemon juice and lemon zest. Set the bowl over a saucepan of simmering water and beat the mixture until it is thick enough to leave a trail when the whisk is lifted, about 15 minutes. Let cool slightly, then strain into a blender. Gradually add the butter, blending at high speed until the cream is light and perfectly smooth, about 5 minutes. Scrape down the container as needed. Transfer the cream to a bowl, cover and refrigerate for up to 2 days.