Step 1: Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans. Line bottoms with parchment paper.
Step 2: In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
Step 3: In a large bowl, cream softened butter and 2 cups granulated sugar until light and fluffy (3-5 minutes).
Step 4: Beat in eggs one at a time, ensuring each is fully incorporated. Add 1 teaspoon vanilla extract.
Step 5: Gradually add the cooled melted white chocolate (6 oz) to the butter mixture, beating until well combined.
Step 6: Alternately add the dry ingredients and buttermilk to the wet ingredients, beginning and ending with the dry. Mix until just combined; do not overmix.
Step 7: Gently fold in 1 cup of fresh raspberries. Distribute batter evenly among prepared pans.
Step 8: Bake for 25-30 minutes, or until a wooden skewer comes out clean. Cool in pans for 10 minutes, then invert onto a wire rack to cool completely.
Step 9: For the Raspberry Filling: In a medium saucepan, combine 2 cups raspberries, 1/2 cup granulated sugar, and 1 tablespoon lemon juice. Bring to a simmer, stirring, until raspberries break down (5-7 minutes).
Step 10: Whisk cornstarch with 2 tablespoons cold water to create a slurry. Stir slurry into simmering raspberry mixture. Cook, stirring, until thickened (1-2 minutes).
Step 11: Remove from heat, stir in 1/2 teaspoon vanilla extract. Transfer to a bowl, cover with plastic wrap pressed onto the surface, and chill completely (at least 2 hours).
Step 12: For the White Chocolate Buttercream: Beat 2 cups softened butter until light and fluffy (3-5 minutes).
Step 13: Gradually add 8 oz cooled melted white chocolate to the butter, beating until incorporated.
Step 14: Gradually add powdered sugar, 1 cup at a time, beating until smooth. Add milk/cream 1 tablespoon at a time if too thick. Beat in 1 teaspoon vanilla extract until light and fluffy.
Step 15: Assembly: Place one cake layer on a serving plate. Pipe a buttercream dam around the edge. Spread half of the chilled raspberry filling within the dam.
Step 16: Place the second cake layer, pipe another dam, and spread remaining raspberry filling.
Step 17: Top with the third cake layer. Crumb coat the entire cake with a thin layer of buttercream. Refrigerate for 15-20 minutes to set.
Step 18: Apply remaining buttercream to frost the entire cake smoothly. Garnish with fresh raspberries and white chocolate curls. Store leftovers refrigerated.