Preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking pan with butter or non-stick spray.
Melt 1 cup of unsalted butter in a medium saucepan over low heat. Stir in 2 cups of granulated sugar until well combined. Allow to cool slightly, then whisk in 4 large eggs and 1 teaspoon of vanilla extract.
In a separate bowl, mix together 1 cup of all-purpose flour, 1 cup of unsweetened cocoa powder, 1/2 teaspoon of salt, and 1/2 teaspoon of baking powder. Gradually add this dry mixture to the wet ingredients, stirring gently until just combined.
In a small bowl, mash 1 cup of fresh raspberries and mix in 1/4 cup of raspberry jam until well combined.
Pour half of the brownie batter into the prepared pan. Drop spoonfuls of the raspberry mixture over the batter, then pour the remaining brownie batter on top. Gently swirl the raspberry mixture into the brownie batter using a knife or skewer.
Bake for 25-30 minutes. Check for doneness with a toothpick.
Allow the brownies to cool in the pan for at least 15 minutes before cutting into squares.