Mix the flour, potato flour, sugar, baking powder and salt in a bowl.
Split the vanilla pod lengthways and scrape out the seeds into the bowl.
Add the butter and half the egg.
Rub with your fingertips until the dough forms a ball.
Refrigerate for one hour.
Preheat the oven to 200°C/400°F/gas mark 6.
Place the dough between sheets of baking parchment and roll out until 1cm thick.
Peel off the top layer and cut out hearts with a 5cm cutter and rounds with a 4cm cutter.
Take the shapes with the holes and paint them with egg on the undersides, before pressing them on to the whole shapes.
Fill the holes with jam.
Put on lined baking trays and brush with the remaining egg.
Bake for 10-12 minutes, then cool on a wire rack.
Store in an airtight tin for up to two weeks.