Start by chopping up the butter in cubes and place in a medium sized saucepan (cutting the butter helps the butter to melt evenly and quickly). Melt butter over a low-to medium heat and mix in brown sugar and white sugar until well blended. Set aside to cool slightly (we need to add the eggs soon and we don’t want them to cook).

In a medium sized bowl, sift together the flour, cocoa powder, baking soda and salt and then whisk them together to ensure that they are evenly distributed. Set this aside.

To the butter mixture, add vanilla, egg and egg yolk and beat until light and creamy.

Now add the sifted ingredients until just blended. Check the temperature of the dough with your hand. If it’s still really warm, it may melt the chocolate. In which case, leave in the fridge for a a few minutes to cool slightly. Once it’s cool enough, stir in the chocolate.

Roll the mixture into walnut sized balls (it’s easier to do this if mixture has been chilled in the fridge for several minutes first), place back into the bowl and refrigerate (bowl and all) for a minimum of 15 minutes or ideally over night – as noted in the post below, this helps the flavours to intensify and prevents the cookies from spreading too quickly when cooking.

Once your cookies have chilled, preheat your oven to 165 Degrees Celsius (330 Degrees Fahrenheit) and grease a baking tray or line with baking paper.

Place cookie dough balls onto the baking tray, but do not press down (i.e. leave them as balls). Cookies should be about 8 cm apart.

Bake for 15 minutes. The cookies will look really soft and as though they are not quite done – this is when you need to take them out. They will continue to cook slightly on the tray and will set in a couple of minutes. Place onto a wire rack to cool and then devour. If you have not chilled the dough, then only bake for 10-12 minutes.