Mix sugar, egg yolks, and passionfruit syrup in a saucepan over low heat.
Cook the mixture until thickened, stirring constantly to avoid scrambling the eggs.
Once the mixture is thick, remove from heat. Let it cool slightly, then add the cold butter.
Cover with plastic wrap and refrigerate.
Spread the filling on top of the crust and decorate with fruits such as strawberries, blueberries, etc.