Beat the egg whites with salt until stiff, then gradually add sugar until glossy. Separately, whisk egg yolks with sugar until light and creamy. Fold in the flour and cocoa powder in layers with the egg whites, then pour into the prepared pan.

Bake the sponge in a preheated oven at 180°C for about 20 minutes. Allow it to cool slightly in the pan, then transfer to a cooling rack. Slice horizontally into two layers and drizzle the base with half of the milk.

Melt the white chocolate over a bain-marie. Whip mascarpone with cream until stiff, then fold in the melted chocolate. This creates a silky, luscious layer for the cake.

Halve the blackberries to ensure even layering and to release their flavor more fully.

Spread half of the mascarpone cream on the sponge base, add half of the blackberries. Place the top sponge layer, drizzle remaining milk, then cover with remaining cream and blackberries. Chill for about 1 hour.