Prepare the Baking Pan: Preheat the oven to 350°F. Grease an 8 × 8-inch baking pan, then line it with parchment paper on all sides. Set the prepared pan aside.
Combine the Cocoa Mixture: Place the melted butter, Dutch-process cocoa powder, and vanilla extract in a mixing bowl. Whisk until thoroughly combined and no lumps of cocoa remain. Set aside.
Whip the Eggs: Add the eggs and packed light brown sugar to a stand mixer fitted with the whisk attachment. Beat until the mixture becomes light, fluffy, and pale in color.
Combine the Mixtures: Add the butter-and-cocoa mixture to the whipped egg mixture. Use a rubber spatula to gently fold everything together until evenly incorporated.
Add the Dry Ingredients: Add the all-purpose flour and salt to the mixture. Fold gently until no streaks of flour remain, taking care not to overmix.
Fold in the Raspberries: Gently fold the chilled raspberries into the brownie batter.If using chocolate chips or chopped chocolate, fold them in at the same time, distributing them throughout the batter without breaking up the berries unnecessarily. Fill the Pan: Transfer the finished batter to the prepared 8 × 8-inch pan and spread it evenly.
Bake the Brownies: Bake at 350°F for roughly 25 minutes, beginning to check for doneness at 20 minutes. The brownies are ready when the edges are set while a knife inserted into the center still comes out with some brownie batter on it. Depending on the oven, they may require up to 35 minutes to cook through.
