Step 1: Preheat your oven to 350°F (175°C).
Step 2: Line a 9x13-inch baking pan with parchment paper.
Step 3: In a medium saucepan over low heat, melt the unsalted butter. Let it cool slightly.
Step 4: In a large mixing bowl, whisk together melted butter and granulated sugar. Add eggs one at a time, whisking well. Mix in vanilla extract.
Step 5: In another bowl, sift together flour, salt, baking powder, cocoa powder, and dark cocoa powder.
Step 6: Gradually add the dry mixture to the wet mixture, folding gently until just combined.
Step 7: Fold in chocolate chips until evenly distributed.
Step 8: Pour the batter into the prepared pan and spread it evenly.
Step 9: Bake for 30-35 minutes. Check with a toothpick for doneness.
Step 10: Allow to cool for at least 15 minutes before lifting out and cooling completely on a wire rack.