I always baked from scratch and use a basic victoria sponge recipe (that I have doctored) most of the time.
4oz self raising flour
4oz caster sugar
2 large eggs
vanilla essence to taste (1/2 tsp ish)
1/2 tsp baking powder (this is optional)
2 tbsp buttermilk
cream the butter and sugar together until light and creamy
add the eggs and blend (I use an electric hand whisk)
add the buttermilk and vanilla essence and blend
sieve the flour as you add it along with the baking powder
Make sure it is all mixed in.
This fills an 8" cake tin with ease
Bake at 160 for 30 - 40 mins
You can easily bulk it up in multiples of 2 oz: 2oz of flour, butter and sugar to 1 egg.