Preheat the oven to 350ºF (177ºC). Line an 8" or 9" square baking pan with foil and spray with nonstick spray. Set aside.
In a medium saucepan over medium heat, melt the butter and chopped chocolate, stirring frequently until completely smooth. Allow to cool for about 15 minutes.
Whisk sugar into cooled chocolate mixture until combined. Add the eggs one at a time, whisking after each addition until smooth. Whisk in the vanilla. Add the flour and salt and stir gently until completely combined.
Pour the batter into the prepared pan. Bake brownies for 32-35 minutes or until a toothpick inserted in the center comes out mostly clean. Allow brownies to cool completely before cutting into squares.
When you are ready to cut the brownies, remove the brownies by lifting the foil overhang straight up. Peel back remaining foil and place brownie block onto a large cutting board. Store leftover brownies at room temperature up to 1 week. Brownies freeze well, up to 3 months. Thaw in refrigerator overnight.
*Room temperature eggs: these incorporate into batter much more easily than cold eggs, although this is not absolutely necessary.