In a medium saucepan, combine raspberries, sugar, lemon juice, and lemon zest. Cook over medium heat for about 10 minutes until raspberries soften and release juices. Strain mixture to remove seeds, returning liquid to saucepan. Mix in cornstarch, stir until thickened, and chill in the refrigerator for at least an hour.

Combine the filling ingredients, cooking and straining as needed.

Preheat oven to 325°F (163°C) and grease cake pans. Cream together butter and sugar until light and fluffy. Mix in egg whites, then alternately add cake flour and sour cream until combined. Bake for 40-42 minutes.

Prepare and bake the cake layers, then let cool.

Dissolve sugar in boiling water for simple syrup. Cool and brush over cake layers.

Beat butter, lemon juice, vanilla extract, and zest until smooth. Gradually add powdered sugar, then mix in melted white chocolate until fluffy.

Layer cake by spreading raspberry filling on the first layer, brushing syrup on the second layer, and covering with white chocolate buttercream. Chill for at least 30 minutes before serving.