Melt half the butter in a pan. Mix it with the flour, but do not let it cook.
2
Instead, add the water and immediately afterwards the egg yoke, pepper and salt.
3
Turn the mix briskly until the first sign of boiling. Let it boil briefly then remove from the stove.
4
Add the rest of the butter, and the lemon juice. Naturally the exact quantity of butter remains a personal choice. The more butter, the finer the sauce will be.