I would'nt double a mix, in my experiance its hard to cook a very large cake and it may burn on the top/raw in the middle and sometimes will sink. I would make 2 smaller cakes, you could always stick them together.
I have just made a ladybird cake, where i doubled the mix, it was very hard to cook, took over 1 hour and was dry on the outside and still sticky in the middle. I have still used it (just putting pics on my proffile)
Hi nailpolish - I use this recipe to make a cake that fills a 10x10 inch baking tin that's 3 inches deep - the recipe is dead easy, it cooks okay & it always tastes nice! You can use half plain/half milk chocolate or all milk if you want it to be less chocolatey.
Also included the icing recipe - haven't made it myself but others have & say it's delicious.
1. Heat the oven to 160C/fan 140C/gas 3. Line a 10x10 inch square cake tin.
2. Mix the flour, sugar and cocoa together in a bowl. Melt the chocolate and butter together with 200ml water in a pan and then beat this along with the eggs into the dry mixture. Pour into the cake tin and bake for 1 hour or until a skewer comes out clean. It may crack a little on top but this will be covered by the icing. Cool.
ICING * 400g plain or milk chocolate * 284ml pot single cream * 25g butter * 100-200g icing sugar * cocoa powder for dusting
1. To make the icing, melt the chocolate with the cream and butter until smooth and then cool to a spreadable consistency (wait till it's completely cold or the icing sugar will melt and it doesn't work), beat in enough icing sugar to make the icing opaque and stiff.
2. Slice the cake horizontally into 2 or 3 layers and spread some icing between each layer. Ice the outside of the cake in a thick even layer and smooth the icing down as much as possible, don't worry about the top too much.
If your dds aren't bothered (lucky you - my ds is very specific and demanding about birthday cakes!), I would keep it simple - the icing in the recipe is fairly sticky/fudgey, so you can stick things onto it - or butter icing would do just as well.
Rows of smarties / dolly mixtures / jelly tots would look good and it would be easy to cut
Also, this cake is lovely, but it can come out a bit rounded / cracked on top - so a decorating scheme that allows for you to slice a half-inch or so off the top to even it out a bit if needed, and cover the top with a nice thick layer of butter icing, is a good plan