Take a baking pan and spread a parchment paper or foil on it and leave it aside.
Combine butter, brown sugar and salt in a large container.
Add the right amount of egg and vanilla extract.
Add the flour and cocoa powder and stir until it combined well. (Note that the batter made need to be thicker.)
Mix the chocolate chips with it.
Preheat the oven to 350° F.
Then pour the entire batter into that baking pan and spread evenly with a spoon.
Then bake it in the oven for at least 15-16 minutes. Make sure that everything bakes well, but do not bake it over.
Once baked, let it cool for at least 30 minutes.
After that cut into small pieces and serve.