Making the Vanilla Cake:

1. Preheat your oven to 350°F (177°C). Prepare an 8-inch round cake pan by lining and greasing it.
2. In a bowl, whisk together the flour, baking powder, and salt. Set aside.
3. In another bowl, cream the butter and sugar until fluffy. Incorporate the eggs one at a time, then gradually add the vanilla extract and milk. Mix in the dry ingredients until just combined.
4. Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool on a wire rack.

Creating the Crumb Topping:

1. Toast the flour on a lined baking sheet in the preheated oven for 5 minutes. Allow to cool slightly.
2. Combine the toasted flour with powdered sugar. Cut in the cold butter until the mixture resembles coarse crumbs, then stir in the lemon zest and vanilla extract. Chill in the refrigerator for 1 hour.

Preparing the Lemon Mascarpone Cream Filling:

1. Whip the heavy cream to stiff peaks in a bowl.
2. In a separate bowl, mix the mascarpone cheese, powdered sugar, lemon zest, and lemon juice until smooth. Gently fold in the whipped cream and chill the mixture.

Assembly:

1. Slice the cooled cake horizontally to create two layers. Place the bottom layer on a serving plate.
2. Reserve ¾ cup of the lemon mascarpone cream filling for the exterior. Spread the remaining filling over the bottom layer, then place the second layer on top.
3. Coat the entire cake with the reserved filling, then gently press the crumb topping onto the cake. For a finishing touch, dust the cake with powdered sugar before serving.