Heat the oven to 350°F (175°C). Grease two 8-inch round cake tins and line the bases with parchment paper to prevent sticking.

In a bowl, combine the flour, baking powder, baking soda, and salt. Whisk everything together so the ingredients are evenly distributed.

In a separate bowl, whisk together the sour cream, milk, egg whites, sunflower oil, and vanilla extract until the mixture is smooth and well blended.

Using an electric mixer, beat the softened butter and sugar together for about 3–4 minutes, or until the mixture becomes pale, light, and fluffy.

Gradually add the dry ingredients to the butter mixture, alternating with the wet mixture. Add them in three stages, starting and finishing with the dry ingredients. Mix only until everything is incorporated and the batter is smooth.

Divide the batter evenly between the prepared cake tins and smooth the tops. Bake for 28–32 minutes, or until a skewer inserted into the centre comes out clean.

Leave the cakes in their tins for 10 minutes before carefully turning them out onto a wire rack. Allow them to cool completely before adding the frosting.

Place the white chocolate and heavy cream in a heatproof bowl. Melt gently, stirring until smooth and glossy. Set aside and allow it to cool to room temperature.

Beat the softened butter until creamy. Gradually add the powdered sugar, mixing well after each addition. Stir in the vanilla extract and salt, then pour in the cooled white chocolate mixture. Continue beating until the frosting is light, fluffy, and silky smooth.

Place one cake layer on a serving plate or cake stand. Spread a generous layer of frosting over the top, then place the second cake layer on top. Cover the top and sides of the cake with the remaining frosting, smoothing it evenly. Finish with white chocolate shavings, fresh berries, or sprinkles for decoration if desired.