Start off by beating the butter, icing sugar and vanilla extract together with a wooden spoon until very soft and creamy and then stir in the sieved flour and cornflour to form a soft paste.
Then divide the mixture between the paper cases. Using the back of a teaspoon dipped briefly in water, make a space in the centre of each one to about two thirds of the way down (so that you end up with a deep tartlet case).
Bake the cakes near the centre of the oven for 20 minutes until golden brown – they will still be just slightly soft in the centre but don’t worry about that.
Cool in the tins for 15 minutes, leaving them in their paper cases.
Now fill the centre of each tartlet with jam and leave to finish cooling.
Just before serving sift a light dusting of icing sugar over the top of each one. It doesn’t matter if the icing sugar obscures the jam – it will soon be absorbed so you’ll end up with delicious little red blobs in the centre.