Line a 9x9 inch square tin with baking paper and preheat oven to 160C (fan) / 180C.
In a jug, mix together water, vinegar and vanilla.
In a large bowl mix together gf plain flour, caster sugar, bicarb, salt and cocoa powder.
Pour in the oil and your jug mixture, then mix once more till you have a smooth batter.
Pour into your prepared tin and bake in the oven for about 30 minutes. Poke with a skewer to check if it's done in the middle - if it comes out clean, then it's done. Allow to cool.
For the icing, mix together icing sugar, cocoa powder, water and vanilla until smooth and glossy.
Spread the icing all over the cooled cake, and cover with colourful sprinkles.