Always make sure you have prepared all tools and ingredients before you start mixing: line the baking tin with parchment paper, preheat the oven and get all the ingredients out ready to use. It is easy to get flustered if you feel unprepared!
This brownie mixture, if the butter cools a lot, is really quite a thick one. No need to be alarmed by this, the brownies will turn out wonderful once baked!
Feel free to add in some nuts or dried fruit to the brownie mixture instead of the chocolate chips/chopped chocolate, if you wish.
The most important tip to remember: make sure the whole brownie is COMPLETELY COOL before you cut it. And use a sharp knife to help you get through the crust more easily.