Preheat the oven to 350°F (175°C)
Grease and line an 8×8-inch baking pan with parchment paper
Melt 1/2 cup butter and 1 cup chocolate chips together
Let the melted chocolate mixture cool slightly
Whisk 2 eggs, 1 cup sugar, and 1 teaspoon vanilla extract in a bowl
Stir in the melted chocolate mixture
Add 1/2 cup all-purpose flour, 1/4 cup cocoa powder, and 1/4 teaspoon salt
Mix until just combined
Fold in extra chocolate chips or nuts if desired
Pour the batter into the prepared pan
Spread the batter evenly
Bake for 20 to 25 minutes
Check for doneness with a toothpick; it should come out with a few moist crumbs
Cool completely in the pan
Lift out and cut into squares