Line a baking sheet with baking parchment. Set aside. Chop the dark chocolate and place it in a heatproof / microwave-safe bowl. Melt the chocolate over a double boiler or in a microwave. Set aside to cool.
Measure the flour, cocoa powder, corn starch, baking soda and salt in a small bowl. Whisk or sift to remove lumps. Set aside.
Place the butter, brown sugar and caster sugar in a large mixing bowl. Beat with an electric mixer (or by hand if you are up to it) on medium-high speed until pale in colour, light and creamy - about 5 minutes. Beat in the egg and vanilla, another 1 minute on medium speed. Scrape in the melted chocolate and beat until fully incorporated. Add in the flour mixture and beat on low speed until incorporated. Use a spatula and scrape the bottom of the mixing bowl a few times to ensure the batter is well-mixed together. Stir in the chocolate chips.
Scoop a heaping tablespoon of cookie dough, roll it into a ball and place it on the lined baking sheet. They don't have to be placed far apart as you are going to refrigerate them first. Refrigerate the cookie dough for 1 hour until firm.
Preheat the oven to 175 degrees Celsius.
Space the cookie dough about 2 inches apart from one another and bake them, straight from the refrigerator and no need to flatten, for 15 to 18 minutes. Place the baking sheet on a wire rack and let the cookies cool completely before storing them in an air-tight container for up to 1 week.