Making the Vanilla Cake:

1. Preheat oven to 350°F (177°C). Grease and line two 8-inch round cake pans with parchment paper.
2. Mix dry ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
3. Cream butter and sugar: In a large bowl, beat butter and granulated sugar until light and fluffy (about 2-3 minutes).
4. Add wet ingredients: Mix in vegetable oil and eggs, one at a time. Stir in vanilla extract.
5. Combine dry and wet ingredients: Gradually add dry ingredients, alternating with milk. Mix until just combined.
6. Bake: Divide batter evenly between cake pans and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
7. Cool completely before assembling.

Making the Crumb Topping:

1. Preheat oven to 350°F (177°C). Spread flour on a baking sheet and bake for 5 minutes to remove raw flour taste. Let cool.
2. Mix dry ingredients: In a bowl, whisk together flour and powdered sugar.
3. Cut in butter: Using a pastry cutter or forks, blend cold butter into the flour mixture until crumbs form.
4. Add flavor: Stir in lemon zest and vanilla extract.
5. Chill in refrigerator for 1 hour.

Making the Lemon Mascarpone Filling:

1. Whip heavy cream: In a bowl, beat heavy whipping cream to stiff peaks. Set aside.
2. Prepare mascarpone mixture: In another bowl, mix mascarpone cheese, powdered sugar, lemon zest, and lemon juice until smooth.
3. Fold in whipped cream gently. Keep refrigerated until ready to use.

Assembling the Cake:

1. Place first cake layer on a serving plate. Spread ¾ of the mascarpone filling over the top.
2. Add second cake layer and spread remaining filling over the top and sides.
3. Press crumb topping gently onto the cake.
4. Chill for at least 1 hour before serving.