Pre-heat oven on gas mark 4 then grease and line a 30 x 20cm rectangular tray.
Add all the ingredients to a bowl and mix them together until fully incorporated. Add the mixture to tray and tap the tin lightly on the side to level the mixture.
Put in the oven for 50 minutes but keep checking on it. Once cooked all the way through, remove from the oven and leave in the tins for 5 minutes.
After 5 minutes, turn out on to a cooking rack and leave to cool completely.
Once the cake is cooled, mix the butter, cream cheese and icing sugar together until incorporated. Then spread the mixture on top of the traybake.