Thought i would put a really easy and quick cake recipe here for you so maybe you dont have to rush out to the shops first thing.
It s basic and you just stick it all in the one bowl and mix then bake -

1 1/2 cups self-raising flour
1 cup sugar
125g butter (softened)
2 eggs (at room temperature)
1/2 cup milk

Put everything in a mixing bowl and mix on low speed till combined
Place in greased pan and bake at 180 degree c
Cook cake until a skewer pierced into the cake comes out clean, (approx. 45mins to 1 hour)
If you want it choclate flavoured - put in about 3 tbs of cocoa

happy birthday