1. Heat the oven to 180C/160C fan/gas 4 and line a muffin tin with the Dr. Oetker Rainbow Cases. Using an electric whisk, beat the butter and sugar in a bowl until pale and fluffy. Beat the eggs in a little at a time, then stir in the Dr. Oetker Madagascan Vanilla Extract. Sift in the flour and Dr. Oetker Baking Powder, then gently fold. Divide the mixture evenly between the cases. Bake for 15-20 mins, or until the cakes are well risen and firm on top. Transfer to a wire rack to cool.
2. To make the chocolate bark, break the Dr. Oetker White Chocolate into chunks and place in a microwaveable bowl. Heat in the microwave for 30 sec bursts, stirring halfway, until smooth and melted. Warm three bowls in the microwave for 1 min, then divide the melted chocolate between the four bowls. Add drops of the different Dr. Oetker Extra Strong Food Colour Gels to three of the bowls and stir, so you have one red, one blue, one orange and one white chocolate.
3. Line a large baking tray with parchment paper and spoon the different coloured chocolate onto it one tablespoon at a time. Once all the chocolate is on the tray, swirl with a cocktail stick to create a marbled effect. Gently tip the tray from side to side to ensure the chocolate has set over an even surface. Decorate with the Dr. Oetker Bright and Bold sprinkles, then leave to set in the fridge. Once the chocolate is cooled, break it into shards.
4. Using a palette knife, spread the Dr. Oetker Vanilla Buttercream Style Icing over the top of each cupcake and decorate with the chocolate shards and a few extra sprinkles.