Step 1: Preheat your oven to 325°F. Mix graham cracker crumbs, 1/4 cup sugar, and melted butter in a bowl. Press the mixture firmly into the bottom of a 9-inch springform pan. Bake for 10 minutes, then remove and let cool.
Step 2: In a large bowl, beat the softened cream cheese and 1 cup of sugar until perfectly smooth and creamy. Add the eggs one at a time, beating on low speed after each addition until just incorporated. Stir in the vanilla extract and heavy cream.
Step 3: Wrap the springform pan in double layers of heavy-duty foil. Pour the cheesecake batter over the crust. Place the pan in a large roasting pan and add enough hot water to reach halfway up the sides of the springform pan.
Step 4: Bake for 70 minutes or until the edges are firm but the center still has a slight jiggle. Turn off the oven, crack the door, and let the cheesecake sit inside for 1 hour. Remove and refrigerate for at least 6 hours.
Step 5: To make the pastry cream, whisk milk, egg yolks, 1/4 cup sugar, and cornstarch in a saucepan over medium heat. Whisk constantly until it thickens into a custard. Remove from heat, stir in vanilla, and let it cool before spreading over the chilled cheesecake.
Step 6: For the ganache, place chopped chocolate in a heatproof bowl. Heat 1/2 cup heavy cream until just boiling, then pour over the chocolate. Let sit for 5 minutes, then stir until smooth and glossy. Pour over the pastry cream layer and chill for another hour before serving.