For the Crust
Preheat your oven to 325°F (165°C).
Mix graham cracker crumbs, melted butter, and sugar in a bowl until combined.
Press the mixture into the bottom of a greased 9-inch springform pan.
Bake for 10 minutes, then let cool completely.
For the Cheesecake Filling
In a large bowl, beat cream cheese and sugar until smooth.
Add eggs one at a time, mixing well after each addition.
Stir in vanilla extract, sour cream, and flour until fully combined.
Pour the filling over the prepared crust and smooth the top.
Bake for 50–60 minutes, or until the center is set but still slightly jiggly.
Allow the cheesecake to cool to room temperature, then refrigerate for at least 4 hours or overnight.
For the Custard Layer
In a saucepan, whisk together sugar and cornstarch. Gradually add milk and whisk until smooth.
Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil.
Remove from heat and whisk in egg yolks one at a time, then return to heat for 1–2 minutes, stirring constantly.
Remove from heat and stir in butter and vanilla extract.
Let the custard cool slightly, then spread evenly over the chilled cheesecake.
For the Ganache
Heat the heavy cream in a small saucepan until it just begins to simmer.
Pour the hot cream over the chopped chocolate in a heatproof bowl. Let sit for 2–3 minutes, then whisk until smooth and glossy.
Pour the ganache over the custard layer, spreading it evenly to cover.
Final Assembly
Refrigerate the cheesecake for at least 2 more hours to set the ganache.
Release the cheesecake from the springform pan carefully and transfer to a serving plate.