Preheat oven to 350°F. Line three 8-inch cake pans with parchment paper circles and spray with baking spray.

Melt white chocolate into the milk. Combine fully and let it cool for 15-20 minutes.

While the chocolate mixture is cooling, cream the butter and sugar in the large bowl of a stand mixer with the paddle attachment, or a large mixing bowl if using an electric mixer. Add the eggs and vanilla extract, mixing to combine.

Next, add the sour cream, and mix until fully combined.

Mix the flour, baking powder, and salt into the mixture to form the batter.

Now add the cooled white chocolate and milk mixture and mix to combine. Divide the batter between the prepared pans and bake in the preheated oven for 24-28 minutes. Level off the tops of the cakes, if needed.

Add the raspberries and sugar to a medium saucepan and cook over medium heat, stirring frequently, until the raspberries release their juices and break down. This should take about 8-10 minutes.

In a small, separate bowl, whisk together the cornstarch, lemon juice, and water to create a slurry. Add to the raspberry mixture while stirring and continue to cook another 1-2 minutes until thickened and goes from cloudy to clear. The filling will continue to thicken as it cools, so it shouldn't be too thick and paste-like in the pan.

Remove from heat and cool completely before using to fill a cake.

Place the chocolate pieces in a medium bowl and melt fully.

Allow the chocolate to cool for 15-20 minutes until no longer warm to the touch so that it doesn't melt the butter.

Beat the butter using the whisk attachment of your hand or stand mixer, and then mix in the melted and cooled white chocolate.

Next, add the powdered sugar, vanilla extract, and heavy cream to the butter mixture.

Lastly, mix the ingredients together until just combined.

Place the bottom cake layer on a cake plate and pipe white chocolate buttercream frosting around the edge of the cake's top surface.

Fill the inside of the frosting circle with the raspberry filling.

Pipe enough frosting onto the top of the filling to cover the cake layer.

Completely cover the cake layer by spreading the frosting evenly over it using an offset spatula or a butter knife.

Carefully place the second cake layer on top and repeat the steps above, and then finish by placing the third cake layer on top.

Cover the outside of the cake completely with a thin layer of buttercream frosting.

Create a crown of raspberries around the edge of the cake's top surface by gently pressing raspberries into the frosting so that they are all pointing upwards.

Portion and slice the cake, transferring each slice onto a serving plate.