Make the cheesecake: Line a 9-inch springform pan with parchment and spray with nonstick cooking spray. Combine the gelatin with 2 tablespoons water in a small microwave-safe bowl and set aside to soften, about 5 minutes. Beat the cream cheese on medium-high speed in the bowl of a stand mixer fitted with a paddle attachment (or in a large bowl if using a hand mixer) until completely smooth, about 1 minute. Scrape down the sides of the bowl. Add the confectioners' sugar, sour cream, lemon juice, vanilla and a pinch of kosher salt, and beat on medium-high speed until incorporated and smooth, about 1 minute.
Microwave the gelatin until melted, stirring as needed, about 30 seconds. Add the gelatin to the cream cheese mixture and beat on medium-high speed to combine, about 30 seconds. Pour the cream cheese-gelatin mixture into the springform pan and refrigerate until set, about 2 hours.
Meanwhile, make the cake: Preheat the oven to 350 degrees F. Butter the bottoms and sides of a 9-inch cake pan; dust with flour and tap out the excess.
Whisk together the flour, baking powder and fine salt in a medium bowl. Beat the butter, granulated sugar and oil in a large bowl with an electric mixer on medium-high speed until light in color and fluffy, about 5 minutes. Beat in the eggs, one at a time, and then the vanilla, until combined.
Reduce the mixer speed to low and alternate adding in the flour mixture and the milk, adding the flour in three additions and the milk in two, starting and ending with the flour. Mix just until smooth. Pour the batter into the prepared pan and bake until golden on top and a toothpick inserted into the center comes out clean, 35 to 40 minutes. Let cool in the pan for 10 minutes, and then turn out onto a wire rack to cool completely, about 2 hours.
To assemble: Evenly cut the cake in half horizontally with a serrated knife so you have two cakes that are the same height. Remove the sides of the springform pan. Place the bottom layer cake upside down on top of the cheesecake and flip it over so that the cheesecake is now on top of the cake. Remove the parchment paper from the top of the cheesecake and place onto a cake plate or cake stand. Top with the second cake layer and place in the fridge while you make your ganache.
Make the ganache: Place the chocolate in a medium bowl. Bring the heavy cream and butter to a boil in a small saucepan over medium heat. Pour over the chocolate and let it stand for 5 minutes. Stir until completely melted and smooth.
Remove the cake from the fridge and pour the ganache over the top of the cake, smoothing out the top with a small offset spatula and letting it drip over the sides. Refrigerate until the chocolate sets, about 20 minutes. Slice and serve.
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