See note before starting. Adjust oven rack to middle position. Preheat oven to 325 degrees F.
Cover a 13x9-inch baking pan with two folded sheets of aluminum foil – the first sheet will go across one way and the second sheet should go the length of the over way and be perpendicular to the first sheet. Cut the foil long enough to hang over all edges by 3 inches. The foil will be used to lift the cooked brownies from the pan. Spray the foil-lined pan with nonstick cooking spray.
In a medium bowl, combine flour, salt and baking powder. Whisk well.
Melt butter in a large heatproof bowl. Add chopped chocolate and stir until completely smooth and incorporated. If necessary, gently heat the chocolate/butter mixture, on HIGH, in 10-second increments, until chocolate is melted. Stir often.
When chocolate mixture is smooth, gradually whisk in the sugar. Add eggs, one-at-a-time until thoroughly combined. Add vanilla and whisk again.
Add flour mixture in three additions. Fold gently with a rubber spatula after each addition, until batter is smooth and fully incorporated. Stir in the chocolate chips.
Transfer the batter to the prepared pan, spreading it into the corners and smoothing the surface.
Bake 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.
Transfer brownies to a cooling rack and cool to room temperature. Remove cooled brownies from the pan by lifting the foil overhang. Cut into 2-inch squares. Serve immediately or store, in an airtight container, at room temperature, up to 3 days.
NOTE: For the pictures, I used a smaller baking pan, 8x9. This creates thicker and more decadent brownies! If you decide to make yours thicker too, bake time is 40-45 minutes. Test with a toothpick for doneness.