Preheat the oven to 170C/325F/Gas 3.
Break the 125g/4oz dark chocolate into pieces and place into a heatproof bowl set over a pan of barely simmering water. Heat, stirring occasionally, until melted.
Place the flour, cocoa, bicarbonate of soda and salt into a large bowl and mix well.
In another bowl, cream the butter and two sugars together until light and fluffy. Add the melted chocolate and stir well.
Beat in the vanilla extract and crack in the egg, then mix in the dry ingredients. Finally stir in the chocolate chips.
Scoop out 12 equal portions of the mixture - an ice cream scoop and a palette knife are the best tools for the job - and place onto a lined baking sheet about 6cm/2Â½in apart. Do not flatten them.
Transfer to the oven and bake for 18 minutes, then test with a skewer - it should come out semi-clean and not wet with batter. If you pierce a chocolate chip, try again.
Leave the cookies to cool on the baking sheet for 4-5 minutes, then transfer them to a cooling rack to harden as they cool.