Step 1: Prepare the Sponge Cake Base
Preheat oven to 350°F (175°C).
Line the bottom of a 9-inch springform pan with parchment paper and lightly grease the sides.
In a bowl, whisk together flour, baking powder, and salt.
In a separate bowl, beat eggs and sugar until pale and fluffy (about 3 minutes).
Add vanilla and milk, then fold in the dry ingredients.
Pour into prepared pan and bake for 10–12 minutes, or until lightly golden and a toothpick comes out clean.
Let cool completely in the pan.

Step 2: Make the Cheesecake Layer
Lower oven temperature to 325°F (163°C).
In a large bowl, beat the cream cheese and sugar until smooth.
Add eggs one at a time, mixing on low speed. Stir in vanilla, sour cream, and flour.
Pour over the cooled sponge cake in the springform pan.
Bake for 50–55 minutes, or until the center is slightly jiggly but set around the edges.
Turn off the oven, crack the door, and let the cheesecake cool inside for 1 hour.
Then refrigerate for at least 4 hours, or overnight.

Step 3: Make the Vanilla Custard Layer
In a saucepan, whisk together milk, sugar, and cornstarch until smooth.
Whisk in egg yolks.
Cook over medium heat, stirring constantly, until the mixture thickens (about 5–7 minutes).
Remove from heat and stir in vanilla and butter.
Let cool slightly, then spread over the chilled cheesecake in an even layer.
Refrigerate again until set (at least 1 hour).

Step 4: Make the Chocolate Ganache
Heat the heavy cream in a small saucepan until just simmering.
Pour over chopped chocolate in a bowl. Let sit for 2 minutes, then stir until smooth.
Add vanilla and mix well.
Let cool slightly, then pour over the custard layer and spread evenly.
Chill until set, about 30 minutes.

Step 5: Slice & Serve
Run a knife around the edge of the pan to loosen, then release the springform.
Slice with a hot knife for clean edges and serve chilled.