In a large bowl, sift the flour, cornflour and icing sugar together. Rub in the butter with your fingertips until the mixture resembles crumbs. Add the egg yolk and vanilla extract. Gently bring into a ball, wrap in cling film and rest in the fridge for 30 minutes or so, as this buttery biscuit dough is very soft and needs to firm up.
On a very well-floured board, with a floured rolling pin, roll out half the dough to 4mm thick. If the dough is too soft to work with easily, place the board of rolled-out dough into the fridge (room permitting!) to firm up for 30 minutes or so. Cut out three circles at a time using a 15cm baking tin or a plate as a guide. Repeat with the remaining dough. Cut a heart shape from the centre of three of the biscuits.
Place the biscuits on two or three baking sheets lined with baking parchment and, using the end of a paint brush, make a pattern all around the edges. Rest in the fridge or at room temperature for about 30 minutes.
If the jam is not smooth, warm it slightly in a small pan, then press through a sieve to remove any lumps. Let it cool completely. Spread