Pre-heat the oven to 200°C (170°C for a fan oven or Gas Mark 6). Grease the sides and base of 2 x 18cm (7”) sandwich tins and line with baking paper.
Whisk cake mix, 2 eggs, 50g of softened butter and 120mls (8 tablespoons) of water on low speed for 30 seconds. Then 2 minutes at high speed until thick and creamy, or beat for 3 minutes with a wooden spoon or whisk. Pour mix equally between the 2 sandwich tins.
Bake for 15 – 16 minutes in the centre of the oven until the cake has risen and is firm to the touch. Remove from the oven and allow to cool on a wire rack.
Place Chocolate Frosting into a medium sized bowl. Add 10ml (2 tablespoons) of warm water and 75g (3oz) of softened (not melted) butter to the mix. Whisk until smooth and creamy.
When cool, sandwich the two halves of cake together using some of the frosting. Then cover the sides and top with the remaining frosting and decorate with the chocolate eggs.