Melt 15 grams butter in a small saucepan over medium heat. Add the flour and stir to incorporate. Cook the flour for a while, but do not let it brown.
Whisk in the hot water and bring to a boil. Then remove the saucepan from the heat and continue whisking until the sauce is smooth. Add the egg yolk, cream, and lemon juice and mix well while gently reheating.
Pass the mixture through a fine strainer. Season the sauce with salt and grind in a little pepper. Beat in the remaining butter.