Pour water into the pot and let it boil - prepare a water bath.
Put the sour cream, eggs, inside of the vanilla pod, lemon zest, flour and sugar in the second bowl and mix everything together into a uniform mass without lumps. It is best to mix it with a whisk, not with a wooden spoon.
Put the mass into a water bath and whisk until thickened.
When you have a thick mass, remove it from the water bath and let it cool.
Beat the butter with a mixer until foamy, gradually add the cooled "custard" spoon by spoon and whip a smooth cream.