In a medium bowl, mix together egg, flour, sugar, vanilla and salt with a wooden spoon.

Cut in the cold butter with a pastry blender (or a fork - that'll just take longer). I can't emphasize enough that the butter needs to be cold - even frozen will work.

As this point, you'll probably need to use your hands to combine all the ingredients - if the butter starts to get melty due to the warmth of your hands, place the bowl in the fridge for a little bit until it firms up again.

Turn the dough out on a flat surface covered in wax paper. Set another piece of wax paper on top and use a rolling pin to flatten the dough to around ½ cm. Place flattened dough in the freezer for 1 hour.

While the dough is chilling, prepare the chocolate tablets. Place chocolate in a microwave-safe bowl, stirring every 15 seconds or so until melted and smooth. Pour melted chocolate into mold and place the mold in the fridge for an hour.

Remove dough from freezer and use a rectangle cookie cutter to cut out each of the biscuits. Place them back in the freezer for another 15-20 minutes and start preheating the oven to 350º F.

Place biscuits on a baking sheet prepared with a silicon baking mat and bake for 10-15 minutes, or until the edges are slightly browned. Let cool on baking sheet for 5 minutes, then move to cooling rack to cool completely.

When biscuits are completely cooled, remove each of the chocolate tablets from the mold. Melt a little bit more dark chocolate and add a couple drops to the center of each biscuit, then place a chocolate tablet on top.

Store in an airtight container, preferably in the fridge to prevent the chocolate from melting. Enjoy!