Preheat the oven to 350F. Line a baking sheet with parchment paper.
In a large bowl add the butter and sugars and beat on high until creamy. Add in the egg and vanilla and continue to mix until blended. Add in the cocoa powder and beat until smooth.
Place a large wire sieve over the bowl and add in the flour, cornstarch, baking soda and salt. Mix with a spoon so that the dry ingredients fall through the sieve into the batter below. Using a spoon mix the dry ingredients into the wet ingredients. Add in the food colouring and continue to mix until the colouring is uniform throughout. Add more or less food colouring to get to the desired colour or leave it out all together.
Add in the chocolate chips and mix until even through batter. Cover with plastic wrap and chill in the fridge for half an hour or longer if you wish. Max 3 days. This will make it easier to form balls.
Scoop out heaping teaspoons of cookie batter and roll into balls using the palms of your hands. Place evenly on the cookie sheet and bake for 10-12 minutes. Remove from oven and carefully transfer to a wire cooling rack.