In a medium saucepan, whisk together the pineapple puree, sugar, egg yolks, cornstarch, and pinch of salt. Turn heat on to medium and whisk constantly until the mixture thickens (this happens just as it's about to come to a boil, which on my stove took about 5 minutes). Remove from heat and whisk in the butter until melted. Store in a glass jar, sealed, for up to one week.