Puree the raspberries and grapefruit juice in a blender. Strain the mixture through a fine-mesh sieve into a medium saucepan, pushing it through with a rubber spatula. Rinse the sieve.
Whisk the sugar, cornstarch, grapefruit zest and salt into the raspberry puree. Whisk in the egg yolks. Cook the mixture over medium heat, whisking constantly, until it¿s thick like pudding, 5 to 7 minutes. Remove from the heat and whisk in the butter a few pieces at a time until incorporated.
Strain the mixture through the sieve into a small bowl, pushing it through with a rubber spatula. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until completely set, at least 4 hours and up to 5 days.