Pre-heat oven to 180°C/350°F/Gas Mark 4. Blend in mix for carrot cake, water and oil in a bowl fitted with whisk beaters on slow speed for 1 minute. Scrape down and mix for a further 4 minutes on medium speed.
Add grated carrot and mix on slow speed for 1 minute.
Place into a greased and lined baking tin 25cm x 35cm (10" x 14") and bake for 35-40 minutes until risen and golden brown. Allow to cool.
To make the topping, cream the Mascarpone cheese and butter together until smooth. Add the icing sugar and vanilla essence. Beat again until smooth then spread over the top of the cake.