Just bake a cake as per the chocolate birthday cake recipe http://www.mumandsons.com/2016/02/birthday-chocolate-cake.html
For the icing you need:
- 175 gr butter (room temperature)
- 125 gr icing sugar
- 200 gr caster sugar
- 2 tablespoons of water
- a teaspoon of vanilla essence
- 1.5 teaspoons of salt
Put the caster sugar in a small pan with the two tablespoons of water. Heat it (without stirring it) until it becomes a brown caramel (it only takes 3-ish minutes but you need to keep watching it as otherwise it burns easily). Take the caramel off the heat and let it cool down for 45 minutes.
Beat the butter and the salt with an electric whisker for a couple of minutes. Add the salt, the vanilla essence and finally the icing sugar.
Use the buttercream to assemble the cake and ice it.