You can either make the pastry cream the day before which is what I do or make it at least 3-4 hours before you plan on making the cake.
Pre-heat oven to 340F (170C), lightly grease and flour or spray two 8 inch (20cm) round springform cake pans or regular pans.
In a medium bowl sift the flour, salt and baking powder.
In the bowl of a stand mixer with the whisk attachment beat the egg whites until stiff. Set aside.
In a separate large bowl beat the egg yolks and sugar for 15 minutes, add the vanilla, then add the flour a little at a time and beat to combine between each addition, add the hot water and beat to combine. Gently fold in the egg whites.
Transfer the batter to the prepared pans and bake for approximately 30-40 minutes or until a toothpick comes out clean or with a few crumbs attached. Leave in the pan to cool 15-20 minutes, then move to a wire rack to cool completely.
While the cake is cooling make the filling and the sugar water
In a large bowl beat the cream and vanilla until soft peaks form, sift in the powdered sugar and beat until stiff peaks form. Gently fold the pastry cream into the whipped sweet cream until completely combined. Cover the bowl and chill until ready to use.
In a small pot add the water, lemon peel and sugar. Heat until boiling and continue to boil for about 5-10 minutes. Set aside to cool completely. If you are using limoncello then add it after the mixture has cooled.
PUTTING IT TOGETHER
Once the cakes have cooled completely, remove the tops (golden brown parts) from both the cakes to make a flat surface. Slice each cake into 3 layers.
One set of layers remove the golden parts from the sides of the cake. Cut each layer into long narrow strips and cut the strips into small square pieces, set aside. (If you wish you can make the cubes even smaller or some people like to blend the cake to form crumbs and cover the cake with crumbs instead of cubes).
To make it easier to assemble the cake, place the first layer of the cake back into the spring form pan, wet the cake with a some of the sweetened water, then spread about a quarter of the filling on top, place the second layer on top and repeat.
Place the third layer on top, top with some filling, remove the cake from the pan and place on a cake plate. Continue to spread the filling to completely cover the whole cake.
Cover the frosted cake with the small cubes of cake, refrigerate for at least 4-5 hours, overnight is even better. Dust with powdered/icing sugar before serving if desired. Enjoy!
Notes
To make homemade cake/pastry flour - for every cup of all purpose flour, remove two tablespoons and replace with two tablespoons of corn starch. Be sure to sift the flour and cornstarch together a couple of times, this way it is sure to be properly combined and lump free. Cake flour in cakes will give you a very tender texture and fine crumb, it will also help the cake to rise.Make sure the bowl you use for beating the egg whites is completely clean and dry.Don’t toss the trimmed cake pieces, you can use them to make this delicious Coffee Cake!The cake should be stored in an airtight container in the fridge. It will keep for up to two days in the fridge. It’s not advised to freeze as the filling will separate.