Preheat the oven to 350°F (175°C)
Grease or line an 8×8-inch baking pan with parchment paper
Melt 1/2 cup butter
Stir in 1 cup sugar
Add 2 eggs and 1 teaspoon vanilla extract
Mix in 1/3 cup unsweetened cocoa powder
Add 1/2 cup all-purpose flour
Add 1/4 teaspoon salt
Stir until just combined
Fold in 1/2 cup chocolate chips or chopped nuts if desired
Pour the batter into the prepared pan
Spread evenly
Bake for 20 to 25 minutes
Check with a toothpick for moist crumbs
Let cool before cutting into squares