Choc traybake deffo the way to go for birthday cakes. I made one for DS, covered it in choc buttercream and then topped with green dessicated coconut. Put football players on , and hey presto a very easy football pitch.
Blend cocoa and water in a cup. Beat sugar and marg till soft and fluffy, add egss and mix well. pour in chocoalte mixture. Beat in flour and baking powder adding milk till smooth batter.
Pour in to greased 12x9 inch roasting tin and bake for 35-40 mins till springy and skewer comes out clean.
Decorate either cake with light covering of jam and roll over bought fondant. Go to a cake shop, they will have it ready made. Just roll out on the side using icing sugar, roll up on wooden rolling pin and place over cake. Go for decorating madness and use sparkles and glitter to make cake very girly crazy. Get some sparkler candles too.
It is easy i promise and the girls will love it. Feeds the five thousand and cuts up easily too.
THE ONLY THING LG&T didnt say was the oven temp but i guessed and did it at 160 which seemed fine