Melt the chocolate over a double boiler until it's melted, or alternatively, heat the chocolate in the microwave in 30-second increments, stirring each time until it's melted.
Preheat the oven to 325F/160C.
Line two baking trays with parchment paper or a silicone baking mat.
Beat the butter, sugar and vanilla together in a stand or hand-held mixer until light and fluffy.
Add in the beaten egg and mix for 1 minute.
Sift the flour, cocoa, soda and beat into the sugar, egg and butter mixture for 1 minute.
Add the melted chocolate and chopped ginger and fold the mixture until it forms a sticky dough with no streaks of flour or melted chocolate.
Cover the bowl with plastic wrap and refrigerate until the dough is firm enough to handle - about 30-60 minutes.
Remove from the fridge and scoop about 1 tablespoon of dough from the bowl. Roll into a ball and place on the baking tray. You want about 6-8 dough balls on the baking tray to leave enough space for spreading.
Bake in the oven for 15 minutes, or until the cookies are softly set, slightly cracked and puffed up.
Leave for 5 minutes on the baking tray to firm up slightly, then transfer to a wire rack to cool.
These cookies are best eaten the day of, but the dough can be made several hours ahead, or even the day before. Leave in the fridge until they're ready to bake.