Step 1: Preheat your oven to 325°F (160°C). Grease and flour two 8-inch round cake pans and line the bottom with parchment paper. Prepare a water bath for the cheesecake by bringing a kettle of water to a boil.
Step 2: For the Cheesecake Layer: In a large bowl, beat 2 (8 oz) packages softened cream cheese, 1 cup granulated sugar, and ¼ teaspoon salt with an electric mixer on medium speed until smooth and creamy, about 2-3 minutes. Scrape down the sides of the bowl.
Step 3: Add 3 large eggs, one at a time, mixing on low speed until just combined after each addition. Do not overmix. Stir in 3 tablespoons all-purpose flour, ¾ cup sour cream, and 1 teaspoon lemon extract until smooth.
Step 4: Pour the cheesecake batter into one of the prepared 8-inch cake pans. Place the pan inside a larger roasting pan. Carefully pour the hot water from the kettle into the roasting pan until it comes halfway up the sides of the cake pan.
Step 5: Bake the cheesecake for 50-60 minutes, or until the edges are set and the center jiggles slightly when gently shaken. Turn off the oven, crack the door open, and let the cheesecake cool in the water bath for 1 hour. Remove from water bath, let cool completely on a wire rack, then refrigerate for at least 4 hours, or preferably overnight, until firm.
Step 6: For the Lemon Cake Layers: Increase oven temperature to 350°F (175°C). In a medium bowl, whisk together 3 cups cake flour, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt. Set aside.
Step 7: In a large bowl, beat 1½ sticks softened unsalted butter and 2 cups granulated sugar with an electric mixer until light and fluffy, about 3-5 minutes. Beat in 3 large eggs, one at a time, until fully incorporated.
Step 8: In a separate bowl, combine 1¼ cups buttermilk, ¼ cup fresh lemon juice, ¼ cup vegetable oil, 1 tablespoon lemon extract, and the zest of 2 lemons.
Step 9: Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the buttermilk mixture in two additions, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition. Do not overmix.
Step 10: Divide the cake batter evenly between the two remaining greased and floured 8-inch cake pans. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake layers cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.
Step 11: For the Cream Cheese Frosting: In a large bowl, beat 1 cup softened unsalted butter and 8 ounces softened cream cheese with an electric mixer until smooth and creamy, about 2-3 minutes.
Step 12: Gradually add 4 cups sifted powdered sugar, 1-2 tablespoons fresh lemon juice, 1 teaspoon vanilla extract, and a pinch of salt. Beat on low speed until combined, then increase to high speed and beat until light and fluffy, about 3-4 minutes.
Step 13: To Assemble: Place one lemon cake layer on your serving plate or cake stand. Spread a generous layer of cream cheese frosting evenly over the top.
Step 14: Carefully unmold the chilled cheesecake layer and place it on top of the frosting-covered cake layer. Spread another layer of cream cheese frosting over the cheesecake.
Step 15: Place the second lemon cake layer on top. Frost the top and sides of the entire cake with the remaining cream cheese frosting. Decorate with extra lemon zest or thin lemon slices if desired. Refrigerate for at least 30 minutes before slicing for clean cuts. Enjoy!