• Preheat your oven to 350°F (175°C). Grease and flour two 9-inch cake pans and line them with parchment paper circles.
• In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside for later.
• In the bowl of a stand mixer, beat the butter and granulated sugar until light and fluffy. Add the oil and mix until well incorporated. Scrape down the sides of the bowl as needed.
• Add the egg whites to the butter mixture, one at a time, beating on low speed after each addition. Stir in the vanilla extract, lemon zest, and lemon juice.
• Gradually add the flour mixture to the butter mixture, alternating with the milk, beginning and ending with the flour mixture. Mix until just combined.
• Divide the batter evenly between the prepared cake pans. Bake for 30-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.
• While the cake layers are baking, prepare the cheesecake layer. Beat the cream cheese until smooth, then add the granulated sugar and vanilla extract. Mix in the egg yolks and egg whites, one at a time. Pour the cheesecake batter into a springform pan lined with a classic graham cracker crust. Place the springform pan in a large roasting pan and fill it with hot water halfway up the sides of the pan. Bake the cheesecake at 325°F (163°C) for 45-50 minutes, or until the center is set.
• While the cakes and cheesecake are cooling, make the blueberry jam filling. Combine the blueberries, granulated sugar, cornstarch, and lemon juice in a medium saucepan. Cook over medium heat, stirring frequently, until the mixture thickens and the blueberries burst, about 10 minutes. Let it cool completely.
• In a stand mixer, beat the butter until creamy. Gradually add the powdered sugar, 2 tablespoons of the blueberry jam, and vanilla extract, beating until smooth. Add the cream cheese one square at a time until fully incorporated. Scrape down the sides of the bowl and mix until the frosting is soft and fluffy.
• Once the cake layers and cheesecake layer have cooled, place one cake layer on a serving plate. Spread a thin layer of the blueberry jam filling over the top. Place the cheesecake layer on top, then spread another thin layer of blueberry jam. Place the second cake layer on top of the cheesecake.
• Frost the top and sides of the cake with a crumb layer of frosting. Refrigerate the cake for at least 30 minutes to set the frosting. Then, frost the cake with the remaining frosting and refrigerate again for at least 30 minutes before serving.
• To finish, top the cake with the remaining blueberries and any leftover blueberry jam. Serve individual slices with a dollop of whipped cream if desired.
• Enjoy your Blueberry Cheesecake Cake with Jam filling!!

FOR THE SOURDOUGH VERSION:

• Adjust the Ingredients: Take away 50 grams of cake flour and 50 grams of oil. Instead, add 100 grams of sourdough starter or discard.
• Mix the Batter: Mix everything except the baking powder and egg whites. Adjust the flour and oil amounts.
• Let it Rest: Let the batter sit out for 2 hours, or you can put it in the fridge overnight to let it ferment.
• Finish the Batter: After 2 hours or the next day, add the baking powder and mix it in. Then, gently fold in the stiff egg whites.
• Bake the Cake: Follow the baking instructions from the original recipe.