Preheat oven to 325°F. Mix graham cracker crumbs, melted butter, and sugar together. Press firmly into the bottom of a 9-inch springform pan. Bake for 10 minutes, then set aside to cool.

Make the pastry cream by whisking egg yolks, sugar, cornstarch, and milk in a saucepan over medium heat. Stir constantly until thickened, about 5 minutes. Remove from heat, stir in butter and vanilla. Cover surface with plastic wrap and refrigerate until cool.

Beat cream cheese on medium speed until smooth. Add sugar and mix well. Add eggs one at a time, mixing on low after each. Stir in sour cream, vanilla, and flour until just combined.

Pour half the cheesecake filling over the cooled crust. Spread the chilled pastry cream evenly on top. Pour the remaining cheesecake filling over the pastry cream layer.

Bake at 325°F for 55 to 60 minutes until edges are set and center has a slight jiggle. Turn oven off, crack the door, and let cheesecake cool inside for 1 hour. Refrigerate for at least 4 hours.

Heat heavy cream until simmering, pour over chopped chocolate, and let sit 2 minutes. Stir until smooth, then pour over the chilled cheesecake. Spread evenly and refrigerate at least 1 hour before serving.