This carrot cake tray bake is best eaten on the day of baking. After that, as with any cake, the sponge will begin to lose some of its moistness and seem dry. Storing the carrot cake in an airtight container not much bigger than the cake and in the fridge will help. If you found your carrot cake to be dry directly out of the oven then the most likely cause is that you have over baked it. This could either be due to an oven that's too hot or baking for too long. Pay careful attention to your oven temp and cooking times and keep an eye on your sponge in the last 10 minutes as it may be ready a little early. You can make sure your cake is fully cooked by inserting a flat skewer into the centre of the cake - it should come out clean. The third most common cause of a dry sponge is not putting enough oil or other wet ingredient in the mix, so check your quantities were correct. Egg weights can vary a little, so if you usually add a splash of milk to your bakes, feel free to do so with this one.