1. Bring grapefruit juice to a boil in a small saucepan over medium-high heat. Reduce heat and simmer until juice is reduced by half, about 5 minutes. Let cool.

2. Whisk together eggs, yolks, sugar, grapefruit juice, grapefruit zest, and a pinch of salt in a double boiler or a heatproof bowl set over (not in) a pot of simmering water. Cook, whisking constantly, until thickened, 6 to 7 minutes. Remove from heat, and whisk in butter, a few pieces at a time.

3. Pour mixture through a fine sieve into a bowl set in a larger bowl filled with ice water, and stir until cool. Cover surface of curd with plastic wrap, and refrigerate until cold, at least 4 hours and up to 2 days.