Preheat oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides.

Combine chopped unsweetened chocolate and butter in a medium saucepan over low heat, stirring frequently until completely smooth, about 2–3 minutes.

Remove the saucepan from heat and let the chocolate mixture cool for about 2 minutes to prevent the eggs from scrambling.

Stir the granulated sugar into the chocolate mixture until combined, then add the eggs one at a time, stirring well after each addition.

Stir in the vanilla extract until the batter is glossy and well mixed.

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, salt, and baking powder until no lumps remain.

Pour the dry ingredients into the chocolate batter and fold gently until just combined, then fold in the chocolate chips if using.

Pour the batter into the prepared pan, spread evenly, and bake at 350°F (175°C) for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs attached.

Remove the brownies from the oven and let them cool in the pan for at least 15 minutes before lifting out using the parchment overhang and cutting into 16 squares.