Preheat oven to 350 degrees F. Butter the sides and bottom of a 3-quart rectangular baking dish or a 13x9x2-inch baking pan.
Place raspberries in a food processor; cover and blend until smooth. If desired, sieve puree to remove seeds. Set puree aside. In a medium bowl whisk together the flour, cocoa powder, and salt; set aside.
Place the chocolate and butter in the top of a double boiler or a large heat-proof bowl. Set over a pan of barely simmering water (the bottom of the pan should not touch the water). Heat and stir occasionally until the chocolate and butter are completely melted and combined. Remove from heat, keeping pan or bowl over water. Add both sugars; whisk until completely combined. Remove from water. Mixture should be at room temperature.
Add three eggs to the chocolate mixture and whisk until just combined. Add the remaining eggs and whisk until just combined. Add the vanilla and stir until combined. Do not overbeat the batter or your brownies will be cakey.
Sprinkle the flour mixture over the chocolate mixture. Using a spatula (not a whisk), fold until there is just a trace amount of flour mixture visible.
Pour the mixture into the prepared pan and smooth the top with your spatula. Drizzle the raspberry puree over the batter and swirl in with a table knife or thin metal spatula. Bake the brownies for 25 to 30 minutes or until a toothpick inserted near the center comes out with a few moist crumbs, rotating the pan halfway through baking. Cool completely on a wire rack.