For the White Chocolate Cake Layers:

Preheat oven to 350°F. Grease and flour two 9-inch round cake pans. Line bottoms with parchment paper.

Melt white chocolate in microwave at 50% power in 30-second intervals, stirring between each, until just melted. Set aside to cool slightly.

Whisk together cake flour, baking powder, baking soda, and salt in a medium bowl.

Beat butter and sugar until light and fluffy, about 3-4 minutes. Add egg yolks one at a time, beating well after each addition. Beat in melted white chocolate and vanilla extract.

Gradually add flour mixture to butter mixture, alternating with buttermilk (beginning and ending with flour). Mix just until combined.

In a clean bowl with clean beaters, whip egg whites until stiff peaks form. Gently fold egg whites into batter.

Divide batter evenly between prepared pans. Bake for 25-30 minutes, until a toothpick inserted in center comes out clean.

Cool in pans for 10 minutes, then remove to wire racks to cool completely.

For the Raspberry Filling:

Combine raspberries and sugar in a medium saucepan.

Whisk together cornstarch, lemon juice, and water (if using fresh berries). Add to raspberry mixture.

Bring to boil over medium heat, stirring constantly. Reduce heat and simmer for 5-7 minutes, until thickened.

Transfer to bowl and refrigerate until completely cooled and thickened, about 2 hours or overnight.

For the White Chocolate Buttercream:

Melt white chocolate using same method as for cake. Allow to cool until just slightly warm but still fluid.

Beat butter until creamy and smooth, about 2 minutes.

Add melted white chocolate and beat until well incorporated.

Gradually add powdered sugar, alternating with heavy cream. Add vanilla extract and salt. Beat for 3-5 minutes until light and fluffy.

Assembly:

Level cake layers if needed.

Place one layer on serving plate. Pipe a ring of buttercream around edge to create a dam.

Spread raspberry filling within buttercream dam.

Place second cake layer on top.

Apply thin crumb coat of frosting, refrigerate 15-20 minutes, then apply final layer of frosting.

Decorate top with fresh raspberries.