Preheat oven to 190 C. In a large mixing bowl cream together the butter/margarine, sugars and vanilla essence. Add the egg and mix until smooth. In a separate bowl sift the flour, cocoa, baking soda and salt together. Add this dry mixture little by little to the first bowl, stirring well after each addition. Mix until richly coloured, gooey and uniformed throughout. Stir in the chocolate chips. Grease a baking sheet (I prefer to use spray-on stuff and then wipe the excess off with a paper towel). Use a teaspoon to scoop balls of the mix (I have a tiny ice cream scoop about the size of a melon baller which works very well) onto the baking sheet, leaving space between the lumps for them to spread out as they cook. Bake in the preheated oven for 8-10 mins. Because I like them squidgy, I remove the cookies when they still yield slightly to the touch-- remember they will firm up once they cool! For a variation, add 1/2 cup of chopped walnuts, or try making with white chocolate chips for a two-tone effect.