Preheat 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, milk and vanilla extract and mix everything together to make a ball of dough. Dust the table with a little flour. Split the dough in half and put one half on the floured table.
Add the cocoa powder to the second half of the dough and knead the dough to mix well. Break the vanilla dough in half and roll out one piece until it's about 5mm thick.
Using heart cutters, cut out heart biscuits. Using a very small heart cutter, cut small hearts out of the middle of each of the biscuits. Break the chocolate dough in half, then roll out and cut 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 they stick together in the oven. Carry on making the biscuits to any design you choose.
Carefully use a palette knife or cake slice to help you move the biscuits on 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 on to a rack and leave to cool.