Preheat your oven to 160C fan. Prepare a rectangular baking tin - mine is approx 32cm x 21.5cm. I grease the tin and then use baking paper - the baking paper should go over the edges so you can lift out the cake once it's cooled.
In a large mixing bowl, cream together your caster sugar and your dairy free spread (or butter). Cream until lighter and fluffy. I use an electric hand whisk.
Gradually add your eggs and vanilla extract and whisk until combined.
Fold in the gluten free self raising flour and xanthan gum to the mixture (this is a big mixture so make sure you carefully fold all the flour in properly)
Spoon/pour your mixture evenly into the baking tin. Place in the oven for about 60 minutes until fully cooked and golden.
Leave to cool in the tin and then lift out, using the baking paper and place on a cooling rack.
Whilst the cake is cooling you can make your icing. Sieve your icing sugar into a large mixing bowl. Add your vanilla extract and very gradually add a tbsp of water at a time and mix in until it becomes the right consistency. It should be quite thick and glossy, don't let it go too runny. If you do sieve in a little more icing sugar.
Once you are happy with the consistency of your icing spoon it over your sponge. Allow it to evenly cover the top and go all the way to the edges.
Sprinkle your coloured strands all over the top of the icing - as many as you like. I use a lot!! Now allow your icing to completely set before you cut into slices. I leave mine about 30 - 60 minutes.
Cut a thin strip off each side (but don't waste, they can be enjoyed!!). Then cut the sponge into squares.
Enjoy as they are or serve up with some custard for a nostalgic school dinner treat!