Place the grapefruit juice in a small saucepan, bring it to a boil, then reduce to a simmer and simmer it until reduced to 3/4 cup. Pour the juice in a measuring cup and allow it to cool to room temperature.
Place a strainer over a large bowl, near the stove. Use a 2 or 3-quart non-reactive saucepan for this curd.
Pour the grapefruit juice into the saucepan. Add 1/2 cup of the sugar and whisk to combine. Taste the juice and then add more sugar to taste. Place the eggs yolks, whole eggs, lemon juice and salt in the saucepan and whisk until it is well blended.
Cook the mixture over medium heat, whisking constantly until the mixture is thickened and is 185 degrees F. It should coat the back of wooden spoon and leave a visible trail when you run your finger through it. When the mixture has thickened and reached temperature remove the saucepan from heat and whisk in the butter 1 tablespoon at a time.
Pour the grapefruit curd into the strainer and press it through the strainer with a spatula. Zest the grapefruit directly into the curd and gently stir the zest into the warm curd.
Cover the surface of the curd with plastic wrap, gently pressing the plastic wrap onto the surface of the curd. Place the curd in the refrigerator and chill for at least 2 hours.
The curd can be refrigerated for up to 2 weeks and frozen for a year. If you freeze the curd allow it to thaw in the refrigerator 24 hours before ready to use.