Preheat the oven to 350°F. Line an 8×8-inch pan with parchment paper, leaving an overhang for easy removal.
In a saucepan over low heat, melt the butter and 8 oz chocolate together, stirring until smooth. Remove from heat and let cool slightly.
Whisk the granulated sugar and brown sugar into the melted chocolate mixture until glossy and fully combined.
Add the eggs one at a time, whisking well after each addition. This step helps create the dense, chewy texture Starbucks brownies are known for.
Stir in the vanilla extract.
Sift together the flour, cocoa powder, and salt. Gently fold into the batter just until combined. Do not overmix.
Fold in chocolate chips if using.
Spread batter evenly into the prepared pan.
Bake for (30-35) minutes, until the edges are set but the center still looks slightly underbaked. A toothpick should come out with thick, fudgy crumbs, not clean.
Cool completely in the pan (at least 2 hours) before slicing. This is crucial for the signature dense texture.