Preheat the oven to 170°C/325°F. Melt the 125g of dark chocolate in the microwave or in a heatproof dish over a pan of simmering water.

Put the flour, cocoa, bicarbonate of soda and salt into a bowl.

Cream the butter and sugars in another bowl. Add the melted chocolate and mix together.

Beat in the vanilla extract and cold egg, and then mix in the dry ingredients. Finally stir in the chocolate chips.

Scoop out 12 equal-sized mounds - an ice cream scoop and a palette knife are the best tools for the job - and place on a lined baking sheet about 6cm apart. Do not flatten them.

Cook for 18 minutes, testing with a cake tester to make sure it comes out semi-clean. If you pierce a chocolate chip, try again.

Leave to cool slightly on the baking sheet for 4-5 minutes, then transfer them to a cooling rack to harden as they cool. Cookies will last up to 5 days in an air-tight container.