Adjust oven rack to middle position and preheat to 350 degrees F. Lightly grease bottom of a 9-inch springform pan. Line with parchment round, and lightly spray entire pan; set aside.
In a large measuring cup whisk together milk, egg whites, and vanilla; set aside.
In a large bowl whisk together flour, sugar, baking powder, and salt on low speed until combined. Add butter, 1 piece at a time, and mix with an electric mixer until only pea-sized pieces of butter remain about 1 minute.
Pour in half of the milk mixture, and mix on medium-high speed until light and fluffy, about 1 minute. Reduce speed to medium-low, and add remaining milk mixture, and beat until incorporated, about 30 seconds.
Pour batter into prepared pan and smooth top with a rubber spatula. Bake until lightly golden and a toothpick inserted comes out clean, about 45-50 minutes, rotating halfway through baking.
Cool cake cool on a wire rack in the pan for 20 minutes. Remove cake from pan, discarding parchment, and let cool completely on a wire rack, about 2 hours.
MAKE CRUMB TOPPING:
Mix together flour, powdered sugar, and lemon zest in a medium bowl. Cut in butter and vanilla until mixture is crumbly. Cover and place in the refrigerator to chill.
Whip heavy cream until stiff peaks form; set aside. Beat mascarpone cheese, powdered sugar, lemon juice, and lemon zest on medium speed until smooth. Fold in whipped cream until combined.
Split cake horizontally, and place 1 layer on cardboard round. Reserve ½ cup of filling, and use the rest to fill the cake. Place the second layer on top of the filling. Use reserved ½ cup filling to frost just the sides of the cake. Press crumb topping into sides of the cake, and chill at least 1 hour. Dust top of the cake with powdered sugar. Slice and serve.