In a large bowl, beat the softened cream cheese and softened butter until smooth and creamy.

Add the Marshmallow Fluff and vanilla extract. Beat on low speed until just combined, then increase speed to medium-high and beat until light and fluffy.

Use immediately as a frosting or filling.

Prepare the filling: In a saucepan, whisk together milk, sugar, and cornstarch until smooth.

Cook over medium heat, stirring constantly, until the mixture thickens and boils. Boil for 1 minute, then remove from heat.

In a separate bowl, whisk the egg yolks. Gradually temper the hot milk mixture into the egg yolks, whisking constantly.

Pour the tempered mixture back into the saucepan and cook over low heat, stirring constantly, for 2 minutes, or until thickened. Do not boil.

Remove from heat and stir in the vanilla extract.

Gently fold in the Marshmallow Fluff until it is mostly melted and incorporated.

Pour the filling into the prepared graham cracker crust.

Chill for at least 4 hours, or until firm.

Top with whipped cream and a drizzle of chocolate syrup if desired before serving.