Step 1: Make the Vanilla Cake Layers
- Preheat the oven to 350°F (177°C). Line the bottoms of two 8-inch round cake pans with parchment paper and grease the sides with butter or cooking spray.
- In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set this mixture aside.
- In a large mixing bowl or the bowl of a stand mixer, cream the butter and granulated sugar together until light and fluffy. Add the vegetable oil and eggs one at a time, mixing well after each addition. Once the mixture is smooth, mix in the vanilla extract.
- Gradually add the dry ingredients to the wet mixture, alternating with the milk. Start and end with the dry ingredients, mixing until just combined. You should have a smooth and slightly thick batter.
- Pour the batter evenly into the prepared cake pans, smoothing the tops with a spatula.
- Bake for 25-30 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let the cakes cool in their pans for about 5 minutes before turning them out onto a wire rack to cool completely.
Step 2: Prepare the Crumb Topping
- Preheat the oven to 350°F (177°C). Spread the flour out on a parchment-lined baking sheet and bake for 5 minutes to cook the raw taste out of the flour. Let it cool for a few minutes.
- In a large bowl, whisk together the cooled flour and powdered sugar. Use a pastry cutter, fork, or your hands to cut the cold butter into the dry ingredients until it forms small crumbs. The butter should be well-incorporated, and the texture should resemble coarse crumbs.
- Stir in the lemon zest and vanilla extract. Place the crumb mixture in the refrigerator to chill for at least 1 hour before using.
Step 3: Prepare the Lemon Mascarpone Cream Filling
- In a large bowl, whip the heavy whipping cream to stiff peaks, using a hand mixer or stand mixer.
- In another bowl, mix the mascarpone cheese, powdered sugar, lemon zest, and lemon juice until smooth and creamy.
- Gently fold the whipped cream into the mascarpone mixture until fully incorporated. Be careful not to deflate the whipped cream. Chill the filling in the refrigerator until ready to use.
Step 4: Assemble the Cake
- Place one of the cooled cake layers on a serving plate or cake stand. Spread or pipe the lemon mascarpone cream filling evenly on top of the cake layer.
- Place the second cake layer on top. Spread the reserved filling over the top and sides of the cake, creating a smooth layer of cream.
- Press the chilled crumb topping onto the sides and top of the cake, ensuring it adheres to the cream layer.
- For a finishing touch, dust powdered sugar over the entire cake to give it a beautiful, elegant appearance.
- Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld together. The cake will keep in the refrigerator for up to 3 days.