Grease a 9” springform pan and place it on a cookie sheet. Place a rack in the bottom third of your oven, and preheat to 325F.
Whisk together the milk, egg whites, and extracts. Set aside close to your mixer. In the bowl of your stand mixer fitted with the paddle attachment, add sugar, self-rising flour, and salt. Beat briefly to combine evenly. Add the softened butter and about ¼ of the milk/egg white mixture. Starting on low speed, mix until combined, about 1 minute. Scrape the bowl. Increase the speed to medium and beat for 2 minutes to mix in some air and lighten the texture. Add the remaining wet ingredients and the zest and beat for about 30 seconds to blend in the liquid. Scrape bowl and beat a few more seconds.
Scrape batter into prepared pan and bake until well-risen, domed, and golden brown, about 45 minutes. Check after 30 minutes and gently rotate the pan to ensure even baking. Tent foil over the pan if necessary to prevent over-browning. Cake is done when it is just starting to pull away from the sides of the pan and the top springs back when pressed lightly. Remove to a rack to cool for about 20 minutes. Run a thin knife around the inside of the pan to release the cake and then remove the springform sides. Allow cake to cool completely.
Lemon Cream Filling and Frosting
Combine all the filling ingredients in the bowl of your stand mixer fitted with the whisk attachment. Whip until the mixture just reaches stiff peaks. Scrape the bowl as necessary and, if using mascarpone, take care to not overwhip or the filling will become grainy. Refrigerate until ready to assemble the cake.
Combine all the crumb ingredients in a medium bowl. Using your hands, mix/crumble together until there is no loose flour and none of the crumbs is larger than about the size of a chickpea. Refrigerate until ready to assemble the cake.
Carefully slice the cake in half horizontally with a long serrated knife or with a cake leveler. Place the domed half on a plate. Keep the bottom half on the base of the springform pan and reattach the sides. (Optional) Brush half the limoncello evenly on the bottom layer of cake. Reserve 1 ¼ cups lemon cream filling and spread the remaining filling in a generous layer on top of the bottom layer. (Optional) Brush/drizzle the remaining limoncello on top of the filling so it will soak into the top layer. Place the top layer of cake on top of the filling, and press down gently. Refrigerate for about an hour to chill the filling. Remove the springform sides from the pan.
Spread the reserved frosting in a very thin layer around the sides and on top of the cake. Don’t worry if a little cake shows through—it really will be a very thin layer of icing which will act as glue for the crumbs. Holding the cake at an angle over a tray with one hand, gather up a small handful of crumbs and press them into the side of the cake, letting the excess fall into the tray. Rotate the cake a little and press more crumbs into the side. Continue until you’ve covered all the sides with crumbs. Pour the remaining crumbs evenly on top of the cake. This will be a very generous layer. If you have any thin spots, fill in with any crumbs that fell onto the tray. Press the top crumbs into place and refrigerate until 30 minutes before serving.
Garnish right before serving. Use a fine-mesh sieve to sift powdered sugar evenly over the top of the crumbs.
To make clean cuts when serving, use a long sharp knife with a thin blade and wipe the blade clean between cuts.Store any leftover cake, covered, in the refrigerator.