In a medium saucepan over medium heat, melt the butter and chocolate together, stirring constantly, until completely smooth. Then, remove from heat and pour the butter and chocolate mix into a large mixing bowl. Set it aside to cool slightly for 10 minutes.

Preheat the oven to 350°F. Line a 9x9" baking pan with parchment paper, leaving an overhang around the edges to easily lift out the finished brownies. Set it aside.

Once the butter / chocolate mix has slightly cooled, whisk in the granulated and brown sugar. Then whisk in the eggs, one at a time, whisking well between each egg. Whisk in the vanilla.

Using a rubber spatula, fold in the flour, cocoa powder and salt. Then fold in the chocolate chips.

Pour the mix into the prepared baking pan and bake 30-35 minutes, until a toothpick inserted into the center comes out with only a few moist crumbs (the toothpick shouldn't come out clean, that means the brownies are overcooked).

Place the pan onto a wire rack and allow them to cool completely. Once cooled, remove the brownies from the pan (if frosting, now's the time) and slice them into squares with a sharp knife.

For the optional milk chocolate frosting : sift together the confectioners sugar and cocoa powder to ensure there are no lumps. Set that aside. In a medium bowl, cream the butter using a hand held or stand mixer. Gradually add the sugar / cocoa powder mix and the cream and vanilla in batches, then mix until everything is combined and incorporated. Frost the brownies only once they're completely cooled.

Store brownies in an airtight container at room temperature for up to one week.

Brownies can also be frozen for up to 3 months. To thaw, place them in the refrigerator overnight.