Prepare the Pan and Oven: Preheat the oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper. Lightly grease the parchment so the brownies slide out easily once baked.
Make the Brownie Batter: Whisk together melted butter and granulated sugar in a large mixing bowl until smooth. Add eggs one at a time, stirring well after each, then mix in the vanilla extract until fully combined. The batter should be thick and glossy.
Combine Dry Ingredients: In a separate bowl, mix cocoa powder, flour, salt, and baking powder. Slowly fold this into the wet mixture until the batter is uniform, being careful not to overmix. The chocolate batter should be dense but spreadable.
Prepare Raspberry Swirl: In a small bowl, mash raspberries with sugar and lemon juice. If using frozen berries, thaw and drain any excess liquid first. The raspberry mixture should be slightly chunky to create texture in the brownies.
Assemble the Brownies: Spread half of the chocolate batter evenly in the prepared pan. Drop spoonfuls of the raspberry puree and scatter half of the white chocolate chunks over the surface. Cover with the remaining chocolate batter.
Swirl for Effect: Use a skewer or knife to gently swirl the raspberry and chocolate together. Avoid overmixing to maintain a marbled pattern with streaks of white chocolate and raspberry.
Bake the Brownies: Bake for 30 to 35 minutes, until the edges look set and a toothpick inserted near the edge comes out mostly clean. The center will remain soft and fudgy due to the raspberry and chocolate mix.
Cool and Serve: Allow the brownies to cool completely in the pan so the swirls set and the chocolate firms. Slice into squares and enjoy a perfect combination of rich chocolate, tart berries, and creamy white chocolate in every bite.
