Prepare crust:
Mix graham cracker crumbs, sugar, and melted butter
Press into bottom of springform pan
Bake at 325°F for 10 minutes
Cool completely
Cheesecake layer:
Beat cream cheese and sugar until smooth
Add eggs one at a time
Mix in sour cream and vanilla
Pour over crust
Bake at 325°F for 1 hour
Cool completely
Custard layer:
Whisk milk, sugar, cornstarch in saucepan
Cook until thickened
Temper egg yolks, then add to mixture
Cook until very thick
Cool completely
Spread over cheesecake layer
Chocolate ganache:
Heat cream until just simmering
Pour over chocolate
Let sit 5 minutes, then whisk until smooth
Pour over custard layer
Refrigerate 4 hours before serving