Melt Butter: Over low heat in a saucepan, stirring occasionally. Alternatively, use a microwave in 30-second bursts. Transfer to a large bowl and cool until slightly warm.
Prepare Wet Mixture: To the cooled melted butter, add the sugar and whisk for 20 seconds. Whisk through the egg and vanilla until well combined.
Incorporate Dry Ingredients: Sift in the flour, cocoa and baking soda. Add the salt. Mix until a thick dough forms.
Add Chocolate: Fold in the chopped chocolate.
Chill: Refrigerate dough for 45 minutes.
Preheat Oven: To 356ºF (180ºC) fan-forced. Line a baking sheet with parchment paper.
Form Cookies: Scoop 45g/1.5 oz (or 3 level tablespoons) of dough, roll into tall balls, and place on the tray, spaced 10cm apart. Do not flatten balls.
Bake: 8-10 minutes until edges are set but centers are soft. Remove and tap the tray on your bench top or stove top 3 times to help slightly deflate.
Cool: Let the cookies sit on the sheet for 15 minutes, then move to a wire rack to cool completely. Enjoy!