For the Cake
Preheat the oven to 350°F. Grease and flour two 8 or 9 x 2-inch round cake pans. In a medium bowl, combine cake flour, baking powder, and salt. Set aside.
In a small bowl, stir together 1/4 cup of the sugar with the zest until zest is coated and mixture looks slightly wet.
Transfer lemon sugar to the bowl of a mixer fitted with a paddle attachment; add butter and beat on medium speed until light and fluffy (about 1-1/2 minutes). Add the remaining 1-1/2 cups sugar and beat until smooth (about 1-1/2 minutes). Beat in a quarter of the milk just until blended. On low speed, add the flour mixture alternately with the milk in three batches, scraping the bowl with a rubber spatula; beat just until blended.
In another large bowl, beat the egg whites with a mixer or immersion blender fitted with whisk attachment on medium speed just until foamy. Add the cream of tartar, increase the speed to medium high, and beat until stiff peaks form. Add a quarter of the whites to the batter and gently fold them in with a rubber spatula. Continue to gently fold in the whites, a quarter at a time, being careful not to deflate the mixture.
Divide the batter evenly between the prepared pans and smooth the tops with the spatula. Bake until a pick inserted in the centers comes out clean, 35 to 40 minutes. Let cool in the pans on a rack for 10 minutes. Run a table knife around the inside of the pans and carefully invert each cake out onto the rack. Flip them right side up and let cool completely.
For the Filling
In a small saucepan, combine the raspberries, sugar and lemon juice. Use a spoon or potato masher to break the raspberries down and release their juices then use an immersion blender to puree.
If you'd like a seedless filling, strain the mixture in a fine sieve and return to saucepan.
Heat mixture over medium high heat until it reaches a low boil.
In a small bowl or cup, combine the water and cornstarch. Add to raspberry mixture and stir quickly to thoroughly combine. Continue to heat the mixture at a low bowl; mixture should thicken and begin to look glossy. Cook 2 minutes longer, stirring constantly to avoid scorching on the bottom. Cool completely.
For the Frosting
In a large bowl, beat the butter, vanilla and cream for 30 seconds. Gradually add in the confectioners’ sugar, beating constantly and occasionally stopping to scrape down the sides of the bowl with a spatula, until the frosting reaches the desired consistency. I like a stiffer frosting for this cake.
To assemble the cake
Place one cool cake later on the plate you’re using for serving. Use a sharp spoon or mellonballer to gently remove about 1/4 inch from the top of the cake–avoid scraping off the very edge–what you want to create is a very shallow well across the whole area of the cake that the filling can fill without being squished out by the top cake layer. Spread the filling in the well, taking care not to drag too much.
Place the top layer on the filling and position to be in line with the bottom layer and flat on top. Use a pastry brush or basting brush to sweep crumbs away from the cake surface and serving plate.
Use about 1 cup of frosting to spread thinly over entire cake surface. This is your crumb coat--it should be smooth but it doesn't matter if the crumbs or a bit of leaked filling shows. Refrigerate at least 30 minutes to set the frosting.
Use the remaining frosting to cover the whole thing to your desired thickness of frosting. Smooth the surface with an offset spatula and pipe decorations if desired.