Preheat oven to 180 deg C.
Cream together 6 oz butter at room temperature and 1 1/4 cups caster sugar. The texture should be smooth and creamy.
Add the beaten eggs a little bit at a time and mix very well after each addition.
Sift the flour, salt and baking powder into another bowl.
Fold the flour mixture into the egg mixture, folding it in carefully, to keep the mixture light and fluffy.
Pour into a large round cake tin or two smaller cake tins. Bake for 25-30 minutes until cake is golden.
If you made one large cake, cut it into half. I found the easiest way to do this is with a piece of string slid through the cake - knives just don''t work since the cake will begin to crumble. Either way, let the cake completely cool before cutting.
Combine all the ingredients together and blend very well to make the butter cream. Spread one side of the cake with jam and the other with butter cream.