Put the butter in a bowl. Hard boil the eggs and peel them while still warm. Strain the yolks through a fine sieve onto the butter (the warm yolks will also soften the butter).
Add mustard, salt, pepper and a squeeze of lemon juice and mix well. Then add the curd cheese and sour cream.
Finally, finely chop the cooked egg white (or grate it on a coarse grater) and stir into the spread along with plenty of chives.