Melt the butter with the both brown and white sugar. This can be done in a pan on the stovetop or a bowl in the microwave. Once the butter has just melted, whisk the melted butter and brown and white sugar together for about 1 minute.
Add in the egg and mix well to combine (about 30 seconds)
Sift in the flour, baking soda (bicarbonate of soda) and salt. Mix with a spatula until just combined. Do not over mix. See note 2
Mix in the chopped chocolate pieces. The cookie dough will look “creamy”. This is normal and is because of the melted butter.
This dough is now going to be refrigerated for 15 minutes. This will harden the dough, making it easier for you to scoop onto the baking sheet.
Line a baking sheet with a Silpat silicone mat or parchment paper. See note 3Use a medium-sized ice cream scoop to shape 12 to 14 cookies. Or use a spoon to scoop 12 to 14 cookies onto your lined cookie sheet. Place the shaped cookie dough about 2 inches (5cm) apart. See note 4
You can either roll the scooped cookies into balls. This will give the cookies a smoother baked texture.Alternately, you can just bake them as a scooped cookie (without the additional rolling). Resulting in a rough craggier baked texture. The shaped dough is now going to be refrigerated once more. This time for 30 minutes. see note 5.
Bake for 10 minutes in a preheated oven of 370°F (190°C) 170° (fan).
Remove from the oven once the edges have turned crisp and golden and the middle lightly browned. Do not over bake. See note 6
Leave the cookies on the baking sheet for about 5 minutes before transferring to a wire cooling rack.