Preheat your oven to 350°F (175°C). Combine the graham cracker crumbs, sugar, and melted unsalted butter until well mixed. Transfer into a springform pan, pressing it firmly into the bottom and slightly up the sides. Bake for 8-10 minutes, then cool completely.

Lower the oven temperature to 325°F (160°C). Beat the softened cream cheese until smooth. Gradually add granulated sugar and flour/cornstarch, mixing until combined. Incorporate the eggs one at a time, followed by the sour cream and vanilla extract. Pour into the cooled crust.

Wrap the bottom of the springform pan in aluminum foil and place it in a larger pan filled with hot water. Bake for 60-75 minutes, then cool gradually in the oven for 1 hour before refrigerating for at least 6-8 hours.

Heat the whole milk and sugar in a saucepan until steaming. Whisk egg yolks with cornstarch until smooth. Temper with hot milk, then return to the saucepan and cook until thickened. Transfer to a bowl, cover, and refrigerate for 2-3 hours.

Heat the heavy cream until it simmers. Pour over the semi-sweet chocolate chips and let it sit for a minute before stirring until smooth. Allow to cool slightly.

Spread the cooled pastry cream evenly over the cheesecake. Pour the chocolate ganache on top and refrigerate for 1-2 hours to set. Garnish with chocolate shavings, fresh berries, or whipped cream before serving.