Preheat your oven to 350°F (175°C), and grease three 8-inch round cake pans to prevent sticking. In a mixing bowl, whisk together the all-purpose flour, baking powder, and salt; set this mixture aside. Using a mixer, cream the unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes. Add eggs one by one, mixing well after each addition, and then incorporate the vanilla extract and melted white chocolate until well combined. Gradually mix in the flour mixture with the whole milk, alternating until just combined, retaining a nice, smooth batter.

Divide the batter evenly among the prepared cake pans. Place them in the preheated oven and bake for 25-30 minutes. You'll know they are ready when the tops turn golden and a toothpick inserted into the center comes out clean. Once baked, allow the cakes to cool in the pans for about 10 minutes. Gently remove them from the pans and transfer to a cooling rack to cool completely before assembly.

For the raspberry filling, combine the fresh or thawed raspberries, sugar, lemon juice, and cornstarch in a saucepan over medium heat. Cook this mixture while stirring frequently until it thickens, which should take about 5-7 minutes. Once thickened, remove from heat and let it cool completely.

While the raspberry filling cools, beat the softened butter on medium speed until creamy, about 2 minutes. Gradually add in the powdered sugar, continuing to mix until well combined. Then, incorporate the cooled melted white chocolate and vanilla extract, followed by enough milk to reach your desired consistency for spreading.

Level the cooled cake layers for an elegant presentation. Place one layer on a serving platter and spread an even layer of raspberry filling on top. Repeat this with the second layer, and then top with the final cake layer. Generously frost the top and sides of the cake with your white chocolate frosting. Chill in the refrigerator for about 30 minutes before slicing.