Preparation

• Melt the butter in a 3-quart saucepan over medium heat. Remove the pan from the heat and whisk in the sugar, grapefruit juice, zest, a pinch of salt, and a pinch of ground cardamom, if using. Whisk in the yolks until smooth. Return the pan to medium-low heat and cook, whisking constantly, until the mixture thickens and coats the back of a wooden spoon, 5 to 6 minutes. Do not boil. Strain through a fine-mesh sieve into a bowl, using the wooden spoon to force it through. Let cool to room temperature, stirring occasionally.
• Cover with plastic wrap directly touching the surface. Refrigerate until ready to use, up to 2 weeks.