Start by greasing a 9” springform pan and placing it on a cookie sheet. This will catch any drips. Next, preheat your oven to 325 degrees Fahrenheit. The right temperature is essential for a perfectly baked cake.
In a mixing bowl, whisk together milk, egg whites, and both vanilla and lemon extracts. This mixture will add moisture and flavor to the cake. Set it aside close to your mixer.
Now, in the bowl of your stand mixer fitted with the paddle attachment, combine sugar, self-rising flour, and kosher salt. Beat briefly just to combine everything evenly.
Add the softened butter and about a quarter of the milk and egg white mixture. Start mixing on low speed until combined, which should take around one minute. Scrape down the bowl to ensure everything is mixed well.
Next, increase the speed to medium and beat for about two minutes. This step is crucial as it adds air to the batter, lightening the texture. You'll want to see it getting fluffy!
Once it's fluffy, add the remaining wet ingredients, including the zest. Beat for about 30 seconds just to blend everything together. Scrape the bowl again if needed.
Pour the batter into the prepared pan. Bake it until it’s well-risen, domed, and golden brown, which should take approximately 45 minutes. Start checking after 30 minutes and gently rotate the pan for even baking.
The cake is done when it starts to pull away from the pan's sides, and the top springs back when pressed lightly. Once baked, remove it from the oven and let it cool for about twenty minutes.
Run a thin knife around the inside of the pan to release the cake. Carefully remove the springform sides and allow the cake to cool completely on a wire rack.
While the cake cools, it’s time to make the filling. In the bowl of your stand mixer fitted with the whisk attachment, combine all the filling ingredients. Whip them until you reach stiff peaks, being careful not to overwhip the mascarpone cheese, as it can become grainy.
Refrigerate the filling until you’re ready to assemble the cake. Meanwhile, prepare the crumb mixture in a medium bowl. Mix all the crumb ingredients together using your hands until it resembles small crumbs, and refrigerate it until you need it.
Once your cake is completely cool, carefully slice it in half horizontally with a long serrated knife. Place the domed half on a serving plate.
Keep the bottom half on the base of the springform pan and reattach the sides. If you're using limoncello, brush half of it evenly on the bottom layer of cake. Spread a generous layer of the lemon cream filling on top.
Brush or drizzle the remaining limoncello on top of the filling to soak it into the top layer. Position the top layer of cake on top of the filling, gently pressing it down. Chill in the refrigerator for about an hour to set the filling.
When ready to frost, remove the sides of the springform pan. Spread a thin layer of reserved frosting all around the sides and top of the cake. This layer doesn't need to be perfect, just enough to glue the crumbs.
Gather a handful of crumbs and press them into the side of the cake, allowing excess to fall onto a tray. Keep rotating the cake and pressing more crumbs until all sides are covered. Pour the remaining crumbs generously over the top.
After covering the cake, refrigerate it for 30 minutes before serving. Garnish with a fine-mesh sieve dusting powdered sugar evenly over the top just before serving.
