They sell those silicone cake cases in Asda 12 for about £2.00 and you can also get the paper cases to put inside the silicone bases.
I like the plain old vanilla recipe for a victoria sponge but you can use it for cup cakes too;
cake/fairy cake mixture;
100g self raising flour (sifted!!)
100g soft marg/butter (depending on how low fat you want)
2 eggs
tea spoon of vanilla essance
100 caster sugar
(add chocolate and chocolate powder to meke a chocolate cake)
mix altogether in a large bowl i find that putting it in all in a microwave helpful if you only have a hand whisk as it makes it easier to blend by hand. Pop into individual silicone cases with the paper inside or a buttered fairy cake tin.
For the butter icing to go on top of the cakes you can use 100g of butter and 100g icing sugar mixed together. You can add chocolate cocoa powder to make it chocolate.
btw 100g of butter equates to two large table spoons if you don't have scales.