Put the eggs, butter, sugar and lemon juice and
zest in a heavy-based pan and cook gently over a low
heat, stirring, until the mixture coats the back of
a spoon.
Leave to cool slightly, then whisk in the
yogurt. Place in a shallow freezer container and
freeze until set, stirring occasionally to break up
the ice crystals.
Transfer to the refrigerator 20
minutes before serving.