Set a sieve over a bowl. In a saucepan, whisk together the egg yolks and sugar until blended. Stir in zest, juice, salt and 110g butter and cook, whisking, until it thickens, about 8 minutes. Strain through the sieve and let cool, stirring.
In an electric mixer, beat the 340g butter until light and creamy, about 2 minutes. Gradually add the lemon curd while beating. Beat until creamy, about 2 minutes.