Move an oven rack so that it is in the center of your oven. Then, preheat the oven to 325°F
Grease the bottom of 9-inch springform pan
In a small bowl, whisk together the milk, egg whites, and extracts. Then set this milk mixture aside for now.
In a large bowl, add the flour, sugar, baking powder, and salt. Whisk ingredients together on low speed until combined.
One butter pat at a time, slowly add the butter and mix the butter in with an electric mixer until only small (pea-sized) pieces of butter remain. This should take approx. 1 minute with softened butter.
Add half of the milk mixture in the bowl and use an electric mixer to mix on medium-high speed until light and fluffy. Then, reduce speed to medium-low and add the rest of the milk mixture into the batter. Continue to beat until milk mixture is well incorporated (approx. 30 seconds).
Pour cake batter into the springform pan. Using a rubber spatula, smooth off the top of the cake batter.
Bake for about 45-50 or until lightly golden (rotate the cake 180° about 25 mins in). The cake is down once it can pass the "toothpick test" (when toothpick or skewer is inserted into the middle of the cake and comes out clean).
Let cake cool for about 2 ½ hours (first 20 mins at room temperature, then in your refrigerator).
Lemon Filling
Whip heavy cream until stiff peaks form
In another bowl, add mascarpone cheese, powdered sugar, lemon juice, and lemon zest. Using an electric mixer, beat together on medium speed until smooth.
Fold in whipped cream into cheese mixture until combined. Side aside.
Crumb Topping
In a medium-size bowl, mix together flour, powdered sugar, kosher salt, and lemon zest in a medium bowl.
Cut cold butter into flour mixture until mixture is in pieces no larger than a pea. Refrigerate until ready to frost cake.
Assembling the Cake
When the cake is completely cool, cut the cake in half horizontally.
Spread about 50% of the filling on the bottom half of the cake.
Place about 40% of the lemon filling on top.
Use the remain Lemon Filling, frost the sides of the cake.
Cover the top and sides of the cake with the Crumb Topping (pressing the crumb topping onto the sides to cover).