Turn the oven to 180°C, fan 160°C, gas mark 5. Cut two pieces of baking paper to fit two baking trays.Beat together the softened butter and sugar until pale and creamy.
Add the flour with the milk and using your hands or a spoon mix everything together to make a ball of dough. Sprinkle a little flour onto the table. Split the dough in half and put one half on to the floured table.
Add the cocoa powder to the bowl and using your hands knead the dough to mix in the cocoa powder.Break the vanilla dough in half and roll out one piece until it is about 5mm thick.
Using heart cutters(6.5cm), cut out heart biscuits. Using a very small heart cutter (2.5cm) and cut small hearts out of the middle of each of the biscuits. Break the chocolate dough in half, roll out and cut some small hearts to fit inside the big vanilla hearts – you will need to gently press the small heart into the big hearts to make sure that they stick together in the oven (this is really important so that they cook together). Carry on making the biscuits to any design you choose.
Carefully use a palette knife or cake slice to help you move the biscuits to the tray. Bake for 8 minutes until the biscuits are just golden and cooked – the time will vary depending on how big they are. Put the biscuits onto a cooling rack and leave to cool.