In a small saucepan, bring pineapple juice to a boil over medium-high heat. Cook, stirring occasionally, until reduced by half, about 10 minutes. Let cool completely.

Whisk sugar, egg, egg yolks, lemon juice, and cornstarch into pineapple juice. Bring to a boil over medium-high heat; cook, stirring constantly, until thickened, 2 to 3 minutes. Strain curd through a fine-mesh sieve into a heatproof bowl, discarding any solids. Add butter, and whisk until smooth and combined. Cover with plastic wrap, pressing wrap directly onto surface of curd to prevent a skin from forming. Refrigerate until cool and ready to use, at least 2 hours or up to 3 days.