Adjust oven rack to lower-middle position and preheat oven to 350˚F.
Whisk flour, salt, and baking powder in a small bowl; set aside.
Cover the sheet pan with foil. Spray with cooking spray.
Place the chocolate pieces and the butter in a stainless bowl. Place over a saucepan with 1 - 2 inches of water. Bring to a simmer. Stir the chocolates as they melt. Do not allow the bottom of the bowl to touch the water.
Remove the bowl from the heat and whisk in the cocoa powder, sugar, and vanilla extract.
Add the eggs, 1 at a time, whisking thoroughly between additions, until thoroughly combined.
Add the dry ingredients; whisk until just incorporated. Use a rubber spatula to do the final stirring.
Add the mini-semi-sweet chocolate morsels and stir with a wooden spoon until thoroughly combined.
Pour the batter into the prepared pan, spreading the batter to cover the entire sheet pan. Bake until a toothpick inserted in the center comes out with barely wet crumbs, 30 - 35 minutes.
Cool the brownies on a cooking rack for 5 minutes. use the foil to lift out the brownies. Cool completely on the rack for at least 2 - 3 hours.