Grapefruit Curd

Pour grapefruit juice into a small saucepan over medium-high heat and bring to a simmer. Simmer until juice is reduced to about 3/4 cup, 3 to 4 minutes. Transfer to a measuring cup to cool slightly; stir in grapefruit zest.

Whisk sugar, egg yolks, and egg in a bowl until smooth. Whisk warm juice into egg mixture. Pour back into the saucepan and heat over medium heat, stirring constantly, until the mixture has thickened to the texture of warm pudding and is thick enough to coat your spoon, 8 to 10 minutes.

Pour the curd through a mesh strainer into a clean bowl to remove zest and any bits of cooked egg. Add butter to the warm curd; stir until butter has melted and the curd is smooth. It will continue to thicken as it cools.