Rub 1 tbsp. spoon of zest and squeeze out 2/3 cup lemon juice.
Melt the butter in a thick-bottomed pan, add sugar, corn starch, salt, lemon juice and zest, bring to a boil. Boil for 1 minute.
In a bowl, beat the yolks, pour a little lemon mass into them, pour everything into a saucepan with the remaining lemon mixture, reduce the heat and cook the cream, stirring, until thickened.
Transfer the cream to a jar, cover with a film and store in the refrigerator for up to one week.