Prepare the Oven and Pan: Preheat the oven to 325°F. Lightly spray an 8 × 12-inch baking pan with nonstick spray. If an 8 × 12-inch pan is unavailable, use a 9 × 9-inch pan instead; the brownies will be slightly thicker and may require a little additional baking time. Combine the Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt until evenly combined. Set the mixture aside.
Make the Chocolate Base: Melt the salted butter in a medium saucepan over medium heat. Add the unsweetened cocoa and cook, stirring constantly, until the mixture reaches the consistency of chocolate sauce.
Cool the Chocolate Mixture: Remove the saucepan from the heat and transfer the hot chocolate mixture to a bowl. Allow it to cool slightly before continuing.
Add the Sugar and Eggs: Whisk the granulated sugar into the slightly cooled chocolate mixture. Add the beaten eggs and whisk until the mixture is smooth and well blended.
Add the Dry Mixture: Stir the reserved flour mixture into the chocolate batter until incorporated.
Finish the Batter: Stir in the vanilla extract and chopped milk chocolate pieces, distributing the chocolate throughout the batter.
Fill the Pan: Pour the finished batter into the prepared baking pan and spread it into an even layer.
Bake the Brownies: Bake at 325°F for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Cut: Allow the brownies to cool before cutting them into bars and serving.
