Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan, lining it with parchment paper.

Melt the unsalted butter in a medium saucepan over low heat, then stir in 2 cups of granulated sugar until smooth.

Incorporate the eggs, one at a time, and stir vigorously before mixing in the vanilla extract.

Whisk together the flour, cocoa powder, salt, and baking powder in a separate bowl, then combine with the wet mixture.

Blend 6 oz of fresh raspberries with 2 tablespoons of sugar and 1 tablespoon of lemon juice until smooth; strain through a sieve.

Pour the brownie batter into the prepared pan and spread evenly. Drop spoonfuls of raspberry puree on top.

Gently swirl the raspberry puree into the brownie batter using a knife or toothpick for a marbled effect.

Bake in the preheated oven for 30-35 minutes. Check for set edges with moist crumbs on a toothpick.

Cool the brownies in the pan for about 10 minutes, then slice them into squares and serve.