Place white chocolate chips and 1/4 cup heavy cream in a heat-safe bowl. Microwave for 45–60 seconds and stir until smooth.

Add the blackberry jam and mix until fully combined.

In a separate bowl, whip the remaining heavy cream with powdered sugar, vanilla extract, and salt until soft peaks form.

Gently fold half of the whipped cream into the blackberry mixture, then fold in the rest.

Cover and refrigerate the mousse for at least 2 hours until chilled.

Preheat the oven to 350°F (175°C). Grease and line three 8-inch cake pans with parchment paper.

Cream the butter and granulated sugar in a stand mixer on high speed for about 3 minutes until light and fluffy.

Add the egg whites and mix until combined.

In a separate bowl, whisk together cake flour, baking powder, and salt.

Add half of the dry ingredients to the butter mixture and mix on low speed.

Add sour cream, oil, and vanilla extract, mixing until smooth.

Mix in the remaining dry ingredients.

Divide half of the batter between the prepared pans.

Drizzle blackberry jam over the batter and swirl gently with a knife.

Top with the remaining batter and smooth evenly.

Bake for 32–35 minutes until a toothpick inserted in the center comes out with a few moist crumbs.

Cool the layers for 10 minutes in the pans before removing and letting them cool completely.

Beat butter in a mixer for about 30 seconds until smooth.

Add vanilla extract and salt.

Gradually mix in powdered sugar and lemon juice until fully combined.

Add the blackberry jam and mix until the frosting reaches a smooth consistency.

Place the first cake layer on a cake board.

Pipe a ring of buttercream around the edge to create a barrier.

Spread blackberry jam inside the ring and add a layer of chilled blackberry mousse.

Top with the second cake layer and repeat the filling process.

Add the final cake layer and apply a thin crumb coat of buttercream.

Chill the cake for about 30 minutes.

Apply the final coat of frosting and decorate with buttercream swirls and fresh blackberries.