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

Melt the butter in a medium saucepan over medium-low heat until fully melted; do not let it brown.

Stir in the sugar, cocoa powder, vanilla, and salt into the hot butter until well blended.

Cool the mixture slightly until warm, about 5 to 10 minutes.

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

When thick and well blended, add the flour and beat until very thick and pulling away from the sides of the bowl.

If using, stir in the optional nuts or chocolate chips.

Scrape the batter into the prepared pan and spread evenly.

Bake for 20 to 30 minutes until edges are set and middle is slightly underbaked.

Allow brownies to cool completely in the pan before slicing.