Preheat your oven to 350°F (175°C). Line your 9x13 inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a medium saucepan over low heat, melt the unsalted butter. Once melted, remove from heat and let it cool slightly.
In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined. Add in the eggs one at a time, mixing thoroughly after each addition. Then, stir in the vanilla extract.
In a separate bowl, sift together the all-purpose flour, cocoa powder, dark cocoa powder, salt, and baking powder. This ensures that everything is evenly distributed and helps prevent any clumps.
Gradually add the dry mixture to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix; it’s okay if there are a few lumps.
Fold in the chocolate chips or chunks, distributing them evenly throughout the batter.
Pour the brownie batter into your prepared baking pan, spreading it evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan on a wire rack for about 10-15 minutes. Use the parchment paper to lift them out of the pan, and let them cool completely before slicing into squares.
