Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line a 20cm round cake tin. Combine 100g of the chocolate chips with 1 tablespoon of the dry cake mix in a small bowl.
Mix the remaining cake mix, oil, water, and eggs gently together in a medium bowl and whisk (by hand or electric mixer) for 2-3 minutes, until smooth and creamy. Stir in the cake mix dusted chocolate chips.
Place 75g of the remaining chocolate chips into a microwaveable proof bowl and cook for 1 minute on medium, until just melting. Cook for another 1 minute and stir. If not fully melted, continue for another 30 seconds.
Divide the cake mixture between 2 bowls, with 2/3 in one bowl and 1/3 in another. Mix the chocolate into the bowl with 1/3 of the mixture. Dollop alternative spoonfuls of the cake mixtures randomly into the cake tin. Cut through the mixture 4-5 times with a knife in S shaped curves, creating a marbling effect with the two types of cake mixture.
Bake in the oven for 55-60 minutes or until a skewer inserted into the centre comes out clean. Cool for 10 minutes on a wire rack then remove the cake from the tin cool completely, about 1 hour.
Spread the icing over the top of the cake and decorate with the remaining chocolate chips.