Preheat oven to 350°F and grease three 8-inch round pans.
Whisk flour, baking powder, and salt in a bowl. In a separate bowl, beat butter and sugar until fluffy, about 3 minutes. Add eggs one at a time, then stir in vanilla and melted white chocolate.
Add the dry mixture in three parts, alternating with milk, starting and ending with dry. Mix just until combined.
Divide batter evenly into pans and bake 25–30 minutes until a toothpick comes out clean. Cool 10 minutes in pans, then transfer to wire racks to cool completely.
For the filling, cook raspberries, sugar, and lemon juice over medium heat for 5–7 minutes until syrupy. Stir in cornstarch slurry and cook 2 minutes until thickened, then cool completely.
For the frosting, beat butter until creamy. Gradually add powdered sugar, then stir in cooled melted white chocolate, vanilla, and 2–3 tbsp milk until smooth.
Level cake layers if needed. Place the first layer on a board, spread frosting, then raspberry filling. Repeat with the second layer, then frost the entire cake.
Chill 20–30 minutes to set, then decorate with fresh raspberries, white chocolate curls, or mint leaves.