Preheat oven to 350°. Line 2 (8") cake pans with parchment and grease with cooking spray. In a large bowl, whisk flour, baking powder, and salt.

In another large bowl, using an electric mixer on medium-high speed, beat granulated sugar, butter, and lemon zest until combined. Add oil and beat until smooth and creamy. Add eggs, one at a time, beating well after each addition, then beat in vanilla. Add dry ingredients and beat on medium-low speed until just a few dry streaks remain. Add milk and beat until incorporated. Divide batter between prepared pans; smooth tops.

Bake cake until a tester inserted into the center comes out clean, 18 to 20 minutes. Invert cakes onto a wire rack and let cool completely. Leave oven on.

In a medium bowl, mix granulated sugar and flour. Add butter and cut into flour mixture with your fingertips until mixture holds together when squeezed. Add vanilla and continue to mix with your hands to combine. Crumble mixture onto a small baking sheet.

Bake crumble until edges start to turn golden, 12 to 15 minutes (it will flatten and spread as it bakes). Stir with a rubber spatula to break crumble apart (it will harden as it cools). Break any big pieces with your hands. Let cool completely.

In a large bowl, using electric mixer on high speed, beat cream until stiff peaks form.

In another large bowl, using electric mixer on medium-high speed, beat powdered sugar and mascarpone until smooth. Add lemon zest and juice and beat until combined. Fold in whipped cream.

Level tops of cakes, if needed. Top one cake half with lemon cream filling. Top with other cake half. Press crumble into sides of cake where filling is exposed. Refrigerate until ready to serve.

Dust top of cake with powdered sugar.