Preheat oven to 350 degrees. Line a 9" x 13" baking dish with parchment paper, and spray the top of the parchment paper with cooking spray.
Very finely chop the unsweetened chocolate. Add chocolate and butter to a small bowl. Microwave, in 30 second increments, stirring after each increment, until butter and chocolate are melted (about 1 - 1 1/2 minutes). Set aside.
In a small mixing bowl, stir together flour, salt, and baking powder. Set aside.
In a large mixing bowl, add sugar, eggs, and vanilla extract. Mix with a mixer for 2 minutes until mixture is light and fluffy.
Slowly, add melted chocolate to egg and butter mixture. Mix until thoroughly combined.
Gradually, fold in flour mixture into chocolate mixture just until flour is incorporated.
Pour batter into prepared baking dish and smooth the top of the batter with an offset spatula.
Bake for 30-35 minutes until a toothpick inserted in the center of the brownies comes out clean. Cool completely for 1-2 hours*.