1) Preheat the oven to 140 (120 fan oven), Gas Mark 4.
2) Line two 20cm sandwich tins with baking parchment.
3) Empty the cake mix into a mixing bowl, add the water, oil and eggs. Using a whisk, mix to make a smooth creamy batter.
4) Use a spatula or metal spoon to empty the cake mix evenly into the sandwich tins, smoothing the batter out so it goes all the way to the edges of the tins.
5) Put the tin in the hot oven and bake for 30 minutes, until cooked through.
6) Remove from the oven and leave to cool on a wire rack.