Pre-heat oven to 180°c
Gently whisk the butter to make it soft. Add sugar and mix until fluffy - do not over-whisk.
In a separate bowl lightly whisk the eggs, then add a small amount at a time and whisk into the butter/sugar mix (try not to let it curdle - if it does add a little of the flour). Sift in your flour a little at a time along with the baking powder folding it into the mixture.
Line 2 baking tins, share the mixture evenly between them. Bake for 20mins - but check after 15 using a spike to see if the bake is done (comes out clean).
Leave to cool in the tins, then turn out onto a wire rack until cold. Fill with the jam and cream (or whatever you fancy). Finish with a dusting of icing.