Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line the base and sides of a 20cm round deep cake tin with butter/oil and baking paper.
Beat the cake mix, in a large bowl, together with the eggs, water and oil with an electric hand whisk or mixer for 2-3 minutes until smooth and creamy.
Pour the cake mixture into the greased and lined tin.
Bake in the centre of the oven for 45 minutes, or until a skewer inserted into the centre of the cake comes out clean. Leave to cool in the tin for 10 minutes, then with the handle of wooden spoon, poke holes into the warm cake 2cm apart.
Drizzle the caramel sauce evenly over the top of the cake and let stand until absorbed. Cover and refrigerate for about 2 hours or until chilled.
Whisk the double cream and icing sugar in a medium bowl with an electric hand whisk or mixer, until stiff peaks form. Spread the whipped cream over the top of the cake. Sprinkle with the butterscotch pieces.