Step 1: Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans, or line them with parchment paper circles.
Step 2: In a medium bowl, whisk together the cake flour, baking powder, and salt. Set aside.
Step 3: In a separate bowl, melt the 6 ounces of white chocolate (for the cake) using a microwave in 30-second intervals or a double boiler. Let it cool slightly.
Step 4: In a large bowl with an electric mixer, cream together the softened unsalted butter and 2 cups granulated sugar until light and fluffy, about 3-5 minutes.
Step 5: Beat in the eggs one at a time, incorporating each fully before adding the next. Add the vanilla extract and mix until combined.
Step 6: Gradually add the cooled melted white chocolate (from Step 3) to the butter mixture, mixing until just combined.
Step 7: Alternately add the dry ingredients and buttermilk to the wet ingredients, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition. Do not overmix.
Step 8: Divide the batter evenly among the prepared cake pans. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 9: Let the cake layers cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely.
Step 10: For the Raspberry Filling: In a medium saucepan, combine the raspberries, ½ cup granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the raspberries break down and the mixture thickens slightly, about 5-7 minutes.
Step 11: In a small bowl, whisk together the cornstarch with 2 tablespoons of cold water to create a slurry. Stir the slurry into the raspberry mixture in the saucepan. Continue to cook, stirring constantly, for another 1-2 minutes, until the filling thickens significantly.
Step 12: Remove from heat and stir in the raspberry liqueur, if using. Transfer the raspberry filling to a bowl, cover, and refrigerate until completely cooled and firm, at least 1 hour.
Step 13: For the White Chocolate Cream Cheese Frosting: In a large bowl with an electric mixer, beat the softened cream cheese and 1 cup unsalted butter until smooth and creamy, about 2-3 minutes.
Step 14: Melt the 8 ounces of white chocolate (for the frosting) and let it cool completely. Gradually add the cooled melted white chocolate to the cream cheese mixture, beating until well combined.
Step 15: Sift the powdered sugar into the mixture gradually, beating on low speed until fully incorporated. Add 1 teaspoon vanilla extract. Increase the speed to medium-high and beat for another 2-3 minutes until the frosting is light, fluffy, and smooth. If too thick, add a tablespoon of milk or cream at a time.
Step 16: To assemble: Level cooled cake layers if needed. Place one cake layer on your serving plate. Spread about half of the raspberry filling evenly, leaving a small border.
Step 17: Place the second cake layer on top, gently pressing down. Spread the remaining raspberry filling over this layer.
Step 18: Top with the third cake layer. Apply a thin layer of cream cheese frosting all over the cake (crumb coat).
Step 19: Refrigerate the cake for 20-30 minutes to set the crumb coat.
Step 20: Once chilled, apply the remaining frosting to the top and sides of the cake, smoothing it out. Decorate with fresh raspberries and white chocolate curls. Refrigerate for at least 30 minutes before slicing and serving.