Preheat oven to 325°F. Line and grease four 7 inch pans or 3 8-inch cake pans with parchment rounds.
Add the unsalted butter and granulated sugar into the bowl of a stand mixer. Beat on a high speed with a whisk attachment for a few minutes, scraping down the sides of the bowl as need with a rubber spatula. The mixture will become lighter in color as air is whipped into it.
Add the egg whites, and mix at a medium speed until they’re incorporated.
Sift the dry ingredients (cake flour, baking powder, and salt) into a separate bowl.
Add half of the dry ingredients into the butter mixture, and mix on a low speed until just incorporated.
Add the sour cream, vanilla extract, and oil, and mix at a low speed until combined, scraping the sides and bottom of the bowl as needed!
Mix in the remaining dry ingredients on a low speed.
Divide batter evenly between the prepared cake pans. I like to use a digital kitchen scale to weigh my pans, to make sure my layers will be the same height.
Bake for 40 minutes, or until a toothpick comes out with a few moist crumbs. Rotate pans halfway through to help them bake evenly.
Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.
Place cake layers into the freezer for 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.
Use a serrated knife to level the tops, and trim the sides to remove any caramelization.
Raspberry Cake Filling
If possible, make the raspberry filling in advance! The extra time will allow the filling to thicken and develop its flavor.
Place the frozen or fresh raspberries, granulated sugar, lemon juice and lemon zest, and 1/4 cup of water in a saucepan and heat over medium high heat.
Stir the mixture until it begins to boil.
Lower the heat to medium-low, and allow the filling to simmer for 10-15 minutes.
Turn off heat and remove the pot from the stove. Pour the filling into a metal strainer suspended over a medium sized bowl, and push through using a rubber stapula. Use a decent amount of pressure to really get all the liquid through the sieve. You should be left with about 1/2 cup of seedy pulp.
In a separate small bowl, make a slurry by combine the remaining 1/4 cup of water with the cornstarch. Stir until the cornstarch has fully dissolved into the water.
Add this cornstarch mixture into the strained raspberry filling, and stir until it is incorporated.
Rinse out your pot and pour the filling back into it (you don’t want any residual seeds getting into the filling).
Heat at a medium high heat, and be sure to stir constantly during this stage to prevent the filling from burning.
Cook until the mixture begins to boil, then reduce the heat to medium low.
Continue to stir, and cook for a few additional minutes.
Turn off heat and pour the raspberry filling into a separate bowl to cool. Cover with plastic wrap and place in the fridge for at least 1 hour, or preferably overnight.
Raspberry White Chocolate Buttercream Frosting:
While the cake layers bake and cool, make the white chocolate buttercream frosting.
Beat the butter on a low speed for 30 seconds with a paddle or whisk attachment, until smooth. Slowly add in the powdered sugar, 1 cup at a time. Alternate with small splashes of cream.
Add in the vanilla extract and salt, and beat on low.
Mix in the melted (and cooled) white chocolate on a low speed, and scrape down the sides and bottom of the bowl with a rubber spatula as needed.
Continue to mix until the ingredients are fully incorporated, and the desired consistency is reached.
If the frosting is too thick, add in additional cream (1 teaspoon at a time). If the frosting is too thin, add in more powdered sugar (quarter of a cup at a time).
Stir by hand with a rubber spatula to make the frosting extra smooth, then cover with plastic wrap and set aside. You will mix in the raspberry powder and additional heavy cream once the cake is crumb coated.
To Assemble This White Chocolate Raspberry Cake:
Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.
Add an even layer of white chocolate buttercream on top of the first cake layer. Chill in the freezer for 5 minutes before topping with 1/3 of the raspberry filling (makes it easier to spread the filling!).
Repeat with remaining cake layers.
Spread a thin coat of frosting around the the cake, to fully cover the cake layers.
Smooth using a bench scraper, then chill the cake in the fridge (30 minutes) or freezer (10 minutes) until the frosting is firm to the touch.
Mix the 1/4 cup of raspberry powder and 1/4 cup of heavy cream into the remaining frosting with a rubber spatula.
Add a second, thicker layer of raspberry white chocolate frosting to the cake, and smooth using an icing comb.
Decorate as desired! I chose to add on a raspberry colored white chocolate ganache drip, and top the cake with some edible flowers and raspberries.