Put the eggs, yolks and sugar into a heatproof bowl, and whisk until smooth and fluffy. In a jug, combine the lemon juice, zest and limoncello. Put the egg mixture bowl over a pan of gently simmering water and whisk for 2 minutes until slightly thickened, then pour in ½ the lemon juice mixture. Whisk again until thickened, then pour in the remaining lemon mixture. Keep whisking until thick and glossy, and the mixture holds a thick trail with the whisk. Remove from the heat, then add the butter, whisking, a piece at a time, adding the next piece once the last has completely melted.