Tip the flour and butter into a food processor and whizz to fine crumbs. Add the sugar and yolks then whizz to a smooth dough. Remove one-third of the dough, add vanilla to what’s left and pulse to whizz in. Wrap this dough in cling film and leave at room temperature.
Put the reserved dough back in the processor with the rosewater or dried raspberries, if using, and a drop of food colouring. Pulse and add more colouring to achieve an intense colour. If the dough has become too soft, pulse in a few spoonfuls more flour until a similar consistency to the vanilla dough. Put the dough in the fridge, remembering to remove it 30 mins before you need to roll it out.
Thickly roll out the coloured dough between baking parchment. Use a small 3-4cm heart cutter to stamp out hearts. Keep re-rolling and stamping until all the dough is used. Line up hearts on top of each other and press to form a cylinder, taking care to retain the heart shape. Wrap in cling film and freeze for 3 hrs until solid.
When the heart dough is hard, roll the vanilla dough, using your hands, into
3 cylinders the length of the heart stack. Stick one to each ‘side’ of the hearts, pat together to form a fat cylinder, then gently roll on your work surface until it completely encircles the coloured dough, with the frozen heart dough hidden in the centre. Chill for 2 hrs.
Heat oven to 180C/160C fan/gas 4. Use a sharp knife to slice the log into 1cm-thick discs. Place on trays lined with baking parchment, allowing space for spreading, and bake for 20 mins. Cool on wire racks. Will keep for up to 4 days in an airtight container.