Preheat the oven to 350°F (175°C)
Grease an 8×8-inch baking pan or line it with parchment paper
Melt 1/2 cup unsalted butter
Stir in 1 cup granulated sugar
Add 2 large eggs
Add 1 teaspoon vanilla extract
Mix until smooth
In a separate bowl, whisk together 1/3 cup unsweetened cocoa powder, 1/2 cup all-purpose flour, 1/4 teaspoon salt, and 1/4 teaspoon baking powder
Add the dry ingredients to the wet ingredients
Stir until just combined
Fold in 1/2 cup chocolate chips or chopped chocolate if desired
Pour the batter into the prepared pan
Spread it evenly
Bake for 20 to 25 minutes
Check for doneness with a toothpick
Remove from the oven
Let cool completely in the pan
Cut into squares and serve