Start by making the whipped white chocolate ganache. Add heavy cream to a large glass measuring or medium bowl. Warm in the microwave for 1 minute. Chop the white chocolate and add it to the warmed heavy cream. Let it sit for 2-3 minutes, then stir together until the white chocolate is fully melted and the mixture is smooth. Place the bowl into the refrigerator to cool completely (this will take about 2-3 hours and should be ready by the time the cake is ready to assemble).

Now make the raspberry compote filling. Add raspberries, sugar, and lemon juice to a medium saucepan over medium heat. Cook for 10 minutes, then reduce heat to medium low and continue to cook for 15-20 minutes, stirring occasionally, until the mixture reduces and thickens slightly. Mix together cornstarch and water in a small bowl, then add the cornstarch slurry to the raspberry mixture. Stir for 3-4 minutes, until the raspberries become jammy. Remove from heat and place into a small bowl to cool completely.

Let's get started on the cake. Preheat the oven to 350 degrees F. Brush 3 8-inch pans with melted butter, then dust with flour, ensuring the entire pan is coated.

In a large bowl, whisk together cake flour, baking powder, and kosher salt. Set aside.

Add room temperature butter, vegetable oil, and sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix together on medium speed for 3-4 minutes, until smooth and fluffy.

Scrape down the sides of the bowl with a rubber spatula. Reduce mixer speed to low and begin adding eggs, ensuring each egg is fully incorporated prior to adding the next. Then add all of the egg whites and mix until combined. Add in vanilla bean paste (or extract) and almond extract and mix to combine.

Add half of the dry ingredients to the wet ingredients and mix until just combined. Then add in whole milk, heavy cream, and sour cream. Continue to mix until combined, then add the remainder of the dry ingredients. Mix until smooth.

Divide batter equally among the 3 prepared cake pans and bake on the middle rack of the oven for 28-32 minutes, until a toothpick inserted into the center of each cake comes out with just a few moist crumbs. Allow the cakes to cool in the pan for 10 minutes, then turn over onto a wire rack. Cover with plastic wrap and to cool completely.

While the cake is cooling, make the buttercream. Chop up the white chocolate and add to a small microwaveable bowl. Microwave for 30 seconds. Stir, then microwave again in 15 second increments until fully melted.

Wash out the bowl of the stand mixer and place it back onto the stand mixer with the paddle attachment. Add room temperature butter and beat on medium high speed for 5 minutes. Add in 2 cups of the powdered sugar and the melted white chocolate and beat on low speed, gradually increasing to medium high speed once combined. Beat for about 1 minute, then add the remaining powdered sugar, vanilla bean paste (or extract), whole milk, and 2 tbsp of the cooled raspberry filling. Mix on medium high for 2 minutes then scrape down the sides of the bowl and continue to mix for 30 seconds.

Remove the chilled white chocolate mixture from the refrigerator. Use an electric hand mixer to beat until fluffy and creamy, about 3-4 minutes.

Now let's assemble the cake. Fill a pastry bag with the raspberry buttercream. Place one cake layer onto a cake stand. Spread half of the whipped white chocolate ganache on top. Using the piping bag, pipe a border the perimeter of the cake, then add half of the raspberry filling on top. Repeat with the second layer.

Place the third cake on top then cover the cake with a thin layer on buttercream. Place the cake into the refrigerator to set for 30 minutes. Remove the cake from the refrigerator and cover with the remaining buttercream and top with any leftover raspberries.