Using electric beaters, cream the butter and sugar in a bowl for 2-3 minutes until light and fluffy. Beat in the lemon zest, then sift in the flour and salt. Combine to a smooth paste.
Bring the dough together in your hands (it will be crumbly and soft) and place on a large sheet of parchment. Cover with more parchment and roll out the dough, between the sheets, to about 1.5cm (1⁄2in) thick. Chill for 30 minutes. Preheat the oven to 170°C/Gas Mark 3; line a large baking tray with parchment.
Remove the dough from the fridge and, still between the parchment, roll out further to about 3mm (1/8in) thick. Remove the top layer of parchment and, using a 7cm (23⁄4in) heart-shaped cookie cutter, stamp out as many biscuits as possible; transfer to the baking tray. Next, stamp a 3cm (11⁄4in) circle (use the base of a piping nozzle or cut freehand with a sharp knife) from the centre of half of the hearts. Carefully remove the centres and re-roll with any scraps of dough to make more biscuits.
Scatter the biscuits with the extra 1 tbsp sugar and bake for 15 minutes, or until light golden. Transfer to a wire rack to cool completely. Mix together the jam and cream. Spread 1 tsp over the unsugared side of the whole heart biscuits and top with the stamped-out biscuits. These are best eaten on the day of making (or store the biscuits and jam separately in airtight containers and assemble just before serving).