Makes about 20
Place the butter, sugar and lemon zest in a large bowl and beat with an electric mixer until pale and fluffy. Whisk in the egg yolks one at a time then beat in the salt and vanilla extract.
Stir in the flour and ground almonds to form a stiff dough then bring it together with your hands. Divide the dough in half, shape each half into a disc, wrap in clingfilm and place in the fridge for half an hour to firm up a bit.
Line two baking sheets with baking parchment. Roll one half of the dough out on a lightly floured surface to no more than 5mm (1/4 inch) thick. Use a heart shaped cutter to cut out as many hearts as you can. Use a small heart shaped cutter to cut a heart out of the centre of half of them. Place the whole hearts on one baking sheet and the ones with a cut out on the other (the cut out ones bake slightly faster).
Re-roll the scraps and cut out more hearts. Repeat with the other half of the dough. Place the trays of hearts in the fridge for 20 minutes while you preheat the oven to 170C/325F/gas mark 3.
Bake the trays one at a time for about 10 minutes until lightly golden. Allow the biscuits to cool on the trays for a couple of minutes then transfer to a wire rack to cool completely. Repeat with the other tray.
Once the biscuits have cooled, dust the tops of the cut out ones with icing sugar. Stir the jam really well until it is smooth. Spread a dollop of jam over each of the whole hearts, leaving a small border. Place one of the cut out hearts on top of each whole heart and serve!
The biscuits will be crisp at first but will soften after a while so if you prefer them crisp don't fill them until just before serving (I prefer them slightly soft). Store in an airtight container.