In a large heatproof bowl, combine the sugar, lemon zest and lemon juice with the egg yolks. Set the bowl over a saucepan of simmering water and cook over moderately high heat, stirring constantly, until thickened, about 5 minutes. Remove from the heat and whisk in the butter until melted and incorporated. Strain the lemon curd into a bowl and press a sheet of plastic wrap directly onto the surface. Refrigerate the curd until chilled, about 2 hours.

In a large bowl, using an electric mixer, whip the cream until firm. Fold two-thirds of the whipped cream into the lemon curd and spoon into bowls. Dollop the remaining whipped cream on top and serve.