125²µ/4Β½΄Η³ϊ jam (raspberry, strawberry and blackcurrant all work well)
1 tsp gluten-free baking powder
225g/8oz caster sugar
1 tbsp icing sugar, sifted
1 tsp vanilla extract
250g/9oz gluten-free white self-raising flour
225g/8oz butter, cubed and well-softened, plus extra for greasing
4 free-range eggs