Place the melted butter, cocoa powder, and vanilla in a mixing bowl and whisk to combine until no lumps of cocoa remain. Set aside.
In a stand mixer fit with the whisk attachment, beat the eggs and brown sugar together until light, fluffy and pale in color.
Add the butter mixture to the egg mixture and use a rubber spatula to fold together.
Add the flour and the salt and fold the mixture until no streaks of flour remain. Last, gently fold in the raspberries and chocolate chips (if using)
Transfer mixture into prepared baking pan and bake for roughly 25 minutes, testing for doneness by inserting a knife into the center of the brownies beginning at 20 minutes. You’ll know they’re done when the edge is set, but a knife inserted into the middle still comes out with a bit of brownie batter on it. It might take up to 35 minutes for them to cook through depending on your oven!
Notes
We like to make these with fresh raspberries, but to prevent them from breaking up when you fold them into the batter, we place them in the freezer for an hour to firm up! If you use store bought frozen raspberries they tend to emit more juice which can make your brownies more cakey!