Rub the butter and flour together in a large bowl until the mixture represents fine breadcrumbs then stir in the cocoa powder. In another bowl beat together the sugar, egg and vanilla until smooth, then pour over the flour and butter. Knead it together with your hands to form a fairly soft dough then knead in the chocolate chips. Wrap in cling film and chill in the fridge for 20-30 mins.
Heat oven to 160C/140C fan/gas 3 and line 2 baking sheets with baking parchment. Put the rested dough onto a lightly floured surface and roll it out to about the thickness of a £1 coin. Use a knife, cookie cutter or pizza cutter to chop into shards, or tear it into strips, whatever you feel like. So the biscuits bake evenly, try to make sure they’re roughly the same size, ours were between 5-7cm long and 2-5cm wide. Feel free to re-roll the dough as much as you like so you can use it all up.
Lay the biscuit pieces onto your prepared trays and bake in the oven for 15-17 mins, you may need to do this in 2 batches. Remove from the oven and leave to cool before removing from the parchment. Allow to cool completely on a wire rack before decorating.
To decorate put the chocolate in separate microwavable bowls and heat each one in 30 sec bursts in the microwave until melted, alternatively set the bowls over pans of barely simmering water until the chocolate has fully melted. Lay all the baked biscuits out on the baking parchment or sheets of foil sitting close together and using a teaspoon, drizzle, pour or splat the melted chocolate over the surface. Sprinkle with the chopped stem ginger, chilli flakes and sea salt if you like. Leave to set completely before serving. Keeps in an airtight container for 2-3 days.