Preheat the oven to 325?F (162?C) and line an 8-inch square baking pan with parchment paper or aluminum foil.

Add the butter to a medium saucepan and melt it completely over medium-low heat.

Stir in the sugar, cocoa powder, vanilla, and salt until blended.

Allow the mixture to cool until it is warm, not hot, about 5 to 10 minutes.

Add the eggs one at a time, stirring vigorously after each addition.

Add the flour and beat into the batter until very thick and pulling away from the sides of the bowl.

Stir in the optional mix-ins, if using.

Scrape the batter into the prepared pan and spread it evenly. Bake for 20 to 30 minutes, or until the edges look set and a toothpick inserted into the center comes out with somewhat moist batter.

Let the brownies cool completely in the pan before slicing.