Place 3 cups milk and vanilla bean paste into a medium saucepan. Cook over high heat, stirring often until it just starts to simmer. Remove from heat.
In a medium bowl, whisk remaining 1 cup milk, egg yolks, sugar, and cornstarch.
While whisking vigorously, slowly pour the hot milk into the egg mixture.
Return mixture to saucepan and cook over high heat, whisking constantly, until it thickens and comes to a boil.
Pass through a strainer and into a small bowl. Place plastic wrap directly on top of custard to prevent a skin from forming. Place in refrigerator to cool completely.
Whisk custard to a smooth texture and pipe into cooled cookie cups. Top with fruit if desired.
Store in fridge and serve within two days.