1. Preheat the oven to 325°F. Grease a 9-inch springform pan and wrap the outside with foil if using a water bath.

2. Mix the chocolate cookie crumbs with melted butter until evenly coated. Press the mixture firmly into the bottom of the pan and bake for 10 minutes. Let the crust cool.

3. Beat the cream cheese until smooth. Add the sugar and mix until combined. Add the eggs one at a time, mixing on low speed after each addition.

4. Blend in the vanilla extract, sour cream, and cornstarch until the batter is smooth and creamy.

5. Pour the cheesecake filling over the cooled crust. Place the pan in a larger roasting pan, add hot water around it, and bake for 55 to 65 minutes until the edges are set and the center still has a slight jiggle.

6. Turn off the oven, crack the door open, and let the cheesecake rest inside for 1 hour. Remove it, cool to room temperature, then refrigerate for at least 4 hours or overnight.

7. To make the pastry cream, heat the milk in a saucepan until warm. In a bowl, whisk the egg yolks, sugar, and cornstarch until smooth.

8. Slowly whisk the warm milk into the egg mixture, then return everything to the saucepan. Cook over medium heat, stirring constantly, until thickened. Remove from the heat and stir in the butter and vanilla extract. Cool completely.

9. Spread the cooled pastry cream evenly over the chilled cheesecake and refrigerate for 30 minutes.

10. Heat the heavy cream until just simmering, then pour it over the chopped semi-sweet chocolate. Let it sit for 2 minutes, then stir until smooth.

11. Pour the ganache over the pastry cream layer, letting it drip naturally over the edges if desired.

12. Top with whipped cream and chopped chocolate or candy. Chill for 1 more hour before slicing and serving.