Melt butter then set aside to cool for 10-15 minutes until it's only slightly warm, Do not let it firm up again.
Pour melted butter into a large mixing bowl. Add both sugars, whisk until well combined for 20 seconds. Be sure not too leave it sitting for too long as it will be harder to whisk through your egg in the next step.
Add egg and vanilla, whisk until well combined, about 15 seconds.
Sift over the flour, cocoa, and baking soda. Add the salt and mix until well combined and a soft cookie dough begins to form, it will be thick.
Add chopped chocolate and fold until combined throughout the dough, you may want to use your hands for this step.
Transfer bowl with cookie dough inside to the refrigerator and chill for 30 minutes. If your bowl is too large for the fridge, just wrap your dough in plastic wrap and pop it into the fridge. NOTE: You can also chill your dough overnight, if do you, remove your dough from the fridge 30 minutes before you create the cookies.
Whilst the dough is chilling, preheat the oven to 180ºC (350ºF). Line a large flat baking tray with parchment paper, and set aside.
Remove dough from the fridge.
To create, weigh out each cookie to be 50g (2oz). If you don’t have a food scale, measure each cookie to be 3 level tablespoons. I highly recommended you use a food scale to measure each one.
Place onto the lined tray, leaving as tall balls, do not flatten.
Repeat with remaining dough, leaving enough space between each cookie on the tray to allow for spreading, about 10cm (4 inches) between each cookie.
Bake for 8-10 minutes or until cracks have appeared on top and the centre of each looks slightly under-baked, which is great, that’s what makes them have a gooey middle!
Remove from oven and tap tray again on your bench top/board 3 times to deflate. Allow cookies to cool on the tray for 10 minutes.
Carefully transfer cookies to a wire rack to cool completely. Or allow for them to firm up a little then enjoy a warm cookie whilst the choc chunks are still gooey!
Serve and enjoy!