To make the cake:
Preheat oven to 350 degrees F.
Grease and flour two 9" round cake pans.
In medium bowl, combine flour, baking powder, and salt, and stir to combine.
In small bowl, combine 1/4 cup of the sugar with the lemon zest, stirring until zest and sugar are completely combined.
In large bowl with electric mixer, beat lemon sugar with butter until light and fluffy, about 1 minute.
Add the remaining 1 1/2 cups of the sugar and beat until smooth, about another minute.
Add 1/4 cup of the milk and beat in until just combined.
Then add a quarter of the flour mixture, beating until just combined. Repeat with remaining milk and flour mixture until all of it is added to the batter. Don't over mix.
In a separate bowl, beat the egg whites until foamy.
Add cream of tartar, then beat on until stiff peaks form.
Fold in the egg white mixture gently, 1/3 of it at a time, being careful not to deflate the mixture.
Pour batter into the prepared cake pans, dividing evenly, and smoothing the tops with a spatula.
Bake in preheated oven for 35-40 minutes, until a cake tester comes out clean. Mine took 35 minutes.
Let cool in the pans for 10-15 minutes, then carefully invert each cake onto a cooling rack. Cool completely.
While cakes are cooling, make the filling.
In a small saucepan, combine the raspberries, sugar, lemon juice, and 1 tablespoon of water. With a whisk or fork mash the raspberries down to release some juices.
Heat over medium heat until it reaches a low boil.
In small bowl, mix remaining tablespoon of water with cornstarch.
Add to raspberry mixture and and stir quickly to combine.
Continue heating the mixture at a low boil for another minute, until it is thickened and looking glossy.
While cake and filling are cooling, make the frosting.
Beat butter on high until light and fluffy, about 1 minute.
Add powdered sugar 1/2 cup at a time, beating for 10 seconds after each cup is combined.
Add vanilla, salt, and milk, beating until well combined. Use 3 tablespoons of milk for firmer frosting, and use 4 tablespoons for a little bit thinner of a frosting, whatever your preference.
To assemble the cake, place first cake on cake stand/plate.
If cake is not even, use a sharp knife to cut off any excess to level it out.
Spread raspberry filling over the cake, leaving about 1 inch around the edge so it doesn't spill out the side when you put the second cake on top.
Place second cake on top.
Frost cake to your liking.
Smooth the surface with an offset spatula or pipe decoration, if desired.
Serve and enjoy!