Step 1: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans or two 9-inch round cake pans. Line the bottoms with parchment paper for easy release.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar on medium-high speed until light and fluffy, about 3-5 minutes. Scrape down the sides of the bowl as needed.
Step 4: Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Step 5: With the mixer on low speed, alternately add the dry ingredients in three additions and the buttermilk in two additions, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.
Step 6: Gently fold in the cooled, melted white chocolate until no streaks remain. Divide the batter evenly among the prepared cake pans.
Step 7: Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure the cakes are completely cool before frosting.
Step 8: For the Raspberry Filling: In a small saucepan, combine the raspberries, granulated sugar, and lemon juice. Bring to a simmer over medium heat, stirring occasionally, until the raspberries break down and the sugar dissolves. This will take about 5-7 minutes.
Step 9: Stir in the cornstarch slurry and cook, stirring constantly, for another 1-2 minutes until the mixture thickens. Remove from heat and let it cool completely. The filling will thicken further as it cools.
Step 10: For the White Chocolate Buttercream: In a large bowl, beat the softened unsalted butter on medium speed until creamy, about 2 minutes.
Step 11: Gradually add the sifted powdered sugar, 1 cup at a time, beating until smooth after each addition. The mixture will be thick.
Step 12: Add the heavy cream or milk, vanilla extract, and pinch of salt. Beat on medium-high speed until light and fluffy, about 3-5 minutes.
Step 13: Fold in the cooled, melted white chocolate until fully incorporated and the buttercream is smooth and uniform.
Step 14: To Assemble the Cake: If necessary, level the cooled cake layers with a serrated knife. Place one cake layer on your serving plate or cake stand.
Step 15: Spread about 1/2 cup of white chocolate buttercream evenly over the first layer, leaving a small border. Create a dam around the edge of the cake using a piping bag or spoon to hold the raspberry filling.
Step 16: Fill the dam with half of the raspberry filling. Carefully place the second cake layer on top, pressing gently. Repeat with another layer of buttercream and the remaining raspberry filling.
Step 17: Place the final cake layer on top. Apply a thin crumb coat of buttercream over the entire cake to seal in any loose crumbs. Chill the cake in the refrigerator for 15-20 minutes.
Step 18: Apply the remaining white chocolate buttercream generously and smoothly to the top and sides of the cake. Use an offset spatula or bench scraper for a smooth finish.
Step 19: Garnish the White Chocolate Raspberry Cake with fresh raspberries and white chocolate curls, if desired. Serve immediately or store for later enjoyment.