Put the butter and sugar in a large bowl and cream together.
Beat the egg slightly then add it to the butter and sugar along with the vanilla extract. Mix together.
Sift the flour, cocoa and salt into the bowl. Mix it together. You should get quite a stiff cookie dough. Add the chocolate chips and stir in.
Line a baking tray with parchment paper. Roll the dough into balls and place them on the parchment paper, with plenty of space around each ball of cookie dough. Some of the balls of cookie dough can be frozen at this point to bake later.
Bake the cookies in the oven for 8-9 minutes. Cool on a wire rack.
Nutritional information is approximate and a guideline only.