1. Make the Cake Layer

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch springform pan.
2. Mix Dry Ingredients: In a medium bowl, sift together flour, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: In a large bowl, beat softened butter and granulated sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
4. Combine Wet and Dry Ingredients: Gradually add the dry ingredients to the wet mixture, alternating with milk, beginning and ending with the dry ingredients. Mix until just combined.
5. Bake: Pour the batter into the prepared springform pan and bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. Let the cake cool completely before proceeding.

2. Make the Cheesecake Layer

1. Preheat Oven: Lower the oven temperature to 325°F (160°C).
2. Prepare the Filling: In a large mixing bowl, beat cream cheese and sugar until smooth and creamy. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
3. Incorporate Cream and Custard: Gradually mix in heavy cream, cornstarch, and custard or vanilla pudding. Beat until the mixture is smooth and lump-free.
4. Assemble and Bake: Pour the cheesecake mixture over the cooled cake layer in the springform pan. Smooth the top with a spatula. Bake for 55-65 minutes, or until the cheesecake is set but slightly jiggly in the center.
5. Cool: Turn off the oven and leave the cheesecake inside with the door ajar for 1 hour. This helps prevent cracking. Remove the cheesecake from the oven and let it cool to room temperature before refrigerating for at least 4 hours or overnight.

3. Prepare the Chocolate Ganache

1. Heat Cream: In a small saucepan, heat heavy cream over medium heat until it begins to simmer. Remove from heat immediately.
2. Melt Chocolate: Place chopped chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit for 2 minutes. Stir gently until the chocolate is completely melted and smooth. Stir in vanilla extract.
3. Cool Slightly: Allow the ganache to cool slightly until it thickens but is still pourable.

4. Assemble the Cheesecake

1. Add Ganache: Pour the slightly cooled ganache over the chilled cheesecake, spreading it evenly with an offset spatula. Allow some ganache to drip down the sides for a dramatic effect.
2. Garnish: Decorate with whipped cream, chocolate shavings, or cherries, if desired.

5. Chill and Serve

Refrigerate the assembled cheesecake for at least 1 hour to set the ganache. Slice and serve chilled.