Preheat the oven to 350°F. Liberally spray a 13x9-inch metal non stick baking pan with cooking spray. Set aside.
Melt the unsweetened chocolate in a small microwave safe bowl in 30 second increments. Repeat stirring each time until completely smooth. Set aside to cool slightly.
In a medium size mixing bowl use a hand mixer to cream together the butter, granulated sugar, brown sugar and vanilla until fluffy.
Add the eggs one at a time beating well after each addition. Add the melted chocolate beating just until combined.
In a separate small bowl, use a whisk to sift together all purpose flour and salt.
Gradually add the sifted flour beating just until fully distributed and a batter is formed. Do not over mix. Mix in the chocolate chips by hand.
Spread the batter evenly into the prepared pan. Bake for 35-40 minutes until a toothpick inserted into the center shows moist crumbs.
Cool in the pan on a cooling rack. Dust with powdered sugar before serving, if desired.