Make the vanilla cake mix as directed, and bake in 2 nine-inch cake pans. Cool completely, and remove from the pans.

Lemon Filling: In a medium-size bowl, combine mascarpone cheese and 2 cups powdered sugar with an electric mixer until smooth. Mix in lemon juice. In a large bowl with an electric mixer on high, whip cream until stiff peaks form. Using a spoon, fold cream cheese mixture into the whipped cream; set aside.

Crumb Topping: In a medium-size bowl, combine flour and 1/2 cup powdered sugar. Add butter and dribble in the vanilla extract. Cut cold butter into flour mixture until mixture is in pieces no larger than a pea. Bake the crumbles for 5 minutes at 350 degrees. Refrigerate until ready to frost cake.

Assembling the cake: When the cake is completely cool, spread about half of the filling on the bottom cake layer. Place the second cake on top, and frost the top and sides of the cake with the rest of the Lemon Cream Filling. Cover the top and sides of the cake with the Crumb Topping, pressing it onto the sides to cover. Refrigerate for at least 3 hours.