Brownies are a classic bake sale treat. A combination of unsweetened chocolate and chocolate chips gives these brownies extra chocolate flavor.
Adjust oven rack to lowest position and heat oven to 350 degrees. Make aluminum foil sling for 13-by-9-inch metal baking pan. Spray foil lightly with vegetable oil spray.
Fold 2 long sheets of aluminum foil. One sheet should measure 13 inches wide and the other 9 inches wide. Both sheets should be about 18 inches long.
Lay sheets of foil in pan so that sheets are perpendicular to each other. Let extra foil hang over edges of pan. Push foil into corners and up sides of pan, smoothing foil so it rests against pan.
In medium bowl, whisk together flour, baking powder, and salt.
Place unsweetened chocolate in large zipper-lock plastic bag and seal. Use rolling pin to gently pound chocolate into small pieces.
In large microwave-safe bowl, combine butter and unsweetened chocolate. Heat in microwave at 50 percent power for 1 minute. Stop microwave and stir mixture with rubber spatula to combine. Heat in microwave at 50 percent power until melted, about 1 minute. Remove bowl from microwave and whisk until smooth.
Add sugar, eggs, and vanilla to chocolate mixture and whisk until smooth.
Add flour mixture and use rubber spatula to gently stir until just combined. Stir in chocolate chips. Scrape batter into prepared baking pan.
Place baking pan in oven. Bake brownies until toothpick inserted in center comes out with few moist crumbs attached, 30 to 35 minutes.
The brownies are done when a toothpick inserted in the center comes out looking like this—with just a few crumbs sticking to it.