Preheat oven to 350° F.
In a double boiler (a heatproof bowl set over a pot of simmering water) melt the butter and chocolate, stirring frequently. When melted, remove bowl and set aside to cool slightly. About 5-10 minutes.
While the chocolate is cooling, grease an 8×8 baking pan with butter. Add in a strip of parchment paper cut to fit the pan with a slight overhang. And grease the parchment paper again with a little butter.
To the slightly cooled chocolate, whisk in the sugar and salt until smooth. Once smooth, whisk in the eggs until combined. Using a rubber spatula, fold in the flour (do not over-mix) and then fold in the chocolate chips and optional mix-ins.
Pour batter into greased pan and spread evenly. Bake for 40-50 minutes (bake time will vary on amount of mix-ins used and oven). When done, the brownies will be just set if you jiggle the pan and a toothpick inserted in the middle will come out with moist crumbs, but no wet batter. Be careful not to over bake. They will continue to cook and set up as the cool.
Allow to cool in the pan. Use the parchment overhang to lift the brownies out of the pan and use a sharp knife to cut into squares (I typically do either 9 very large brownies or 16 small brownies).
