Use Stork or the supermarket equivalent for a light and fluffy cake. Butter will gave a denser cake.
If using an 8"/20cm square cake tin, the cake may take a little longer to bake.
Use gel food colouring for the buttercream. Ordinary food colouring won't work as it is too weak. I like to use Sugarflair gel colouring.
Cake will last 4-5 days stored in an airtight container.