Preheat oven to 180°C (160°C fan forced). Grease and line 2, 20cm deep round cake pans. Line the bases with baking paper.
Beat cake mix, eggs, oil and water in a large bowl with electric beaters on low speed for 30 seconds, then on medium speed for 2 minutes. Divide mixture between pans.
Bake for 50 minutes, or until a skewer inserted into the centre of each comes out clean. Cool for 10 minutes in pan, and then gently remove from pans onto a wire rack. Cool completely.
Slice each cake in half through the centre to give four discs. Gently slice off any doming so the discs are flat. Place a disc onto a cake turntable or cake stand and spread with some vanilla frosting. Repeat, ensuring each disc is sitting evenly on top of the one under it. Frost top and whole cake with more vanilla frosting in a thin crumb coat. Chill. Frost whole cake again more generously.
Using a large spatula, scrape back frosting around the side, in a continuous action if you are using a turn table, so you can see the cake. Chill cake for at least 30 minutes, longer if possible until firm to touch.
For the drip: Remove lid and foil seal from chocolate frosting container. Replace lid loosely and heat in microwave on HIGH (1100w) for 30 seconds, stir. Heat for another 10 seconds until quite runny when stirred with a spoon. Time will depend on microwave wattage. Gently pour some of the runny frosting into the centre of the cake and ease over edges with a spatula. Drips will form down the sides of the cake. Repeat, covering the whole top of the cake.