Preheat the oven to 350°F (175°C). Line three 8-inch round cake pans with parchment and grease the sides.

In a small saucepan, combine the fresh raspberries, sugar, and cornstarch. Simmer over medium heat until thickened, about 7–10 minutes. Stir in lemon juice and raspberry jam if using. Let it cool completely.

In a large bowl, cream the butter and sugar until light and fluffy, about 4–5 minutes.

Add in the eggs one at a time, beating well after each. Stir in the vanilla extract.

In a separate bowl, whisk together the all-purpose flour, cake flour, baking powder, and salt. In another bowl, combine the milk and sour cream.

Alternate adding the dry and wet mixtures to the batter, starting and ending with the dry. Do not overmix.

Lightly fold the floured fresh raspberries into the batter.

Divide the batter evenly among the pans and bake for 22–25 minutes or until a toothpick comes out with moist crumbs. Let cool completely on wire racks.

In a large bowl, beat the butter until fluffy. Gradually add the powdered sugar, then beat in the cooled white chocolate, cream, vanilla, and salt until smooth.

Layer the cakes with raspberry filling and white chocolate buttercream. Frost the outside with the remaining buttercream. Garnish with fresh berries and powdered sugar.