Make, bake and cool the cakes as directed on the box using the eggs, butter and milk.
Level tops of cooled cakes. Cut each cake horizontally through the centre to give four discs. Place one disc on a cake decorating turntable, serving plate or cake stand. Spread with one-third of the sour cream and top with one-third of the raspberries. Repeat, ensuring each disc is sitting evenly on top of the one under it.
Frost the top and side of the cake with the vanilla frosting, plus the frosting from the sachets in a thin coat, scraping off the excess with a long palette knife to give a naked effect around the sides of the cake. Chill 15 minutes until firm to touch.
Transfer cake to a serving plate or stand if not already on one.
For the drip, heat the chocolate frosting to tub directions until quite runny when stirred.
Working quickly, gently pour about three-quarters of the runny frosting into the center of the cake and ease over edges with a spatula.
Drips will run down the sides of the cake. Top with the decorations.