In a medium saucepan, combine 1 cup of raspberries, 1/4 cup of sugar, and a splash of water. Cook over medium heat for about 5–7 minutes until the raspberries break down and create a thick, syrupy sauce. Use a fork to mash the mixture slightly, then strain it through a fine sieve for a smooth consistency. Set aside to cool.

Preheat your oven to 350°F (175°C). Prepare an 8x8-inch baking pan by lining it with parchment paper, leaving some overhang for easy removal later.

In a saucepan, combine 1 cup of semi-sweet chocolate chips and 1/2 cup of butter. Melt them together over low heat, stirring constantly for about 3–5 minutes until silky-smooth. Allow to cool for a few minutes.

In a large mixing bowl, whisk together 3 large eggs, 1 cup of granulated sugar, and 1 teaspoon of vanilla extract until well combined and slightly frothy.

Sift together 1/2 cup of all-purpose flour, 1/4 teaspoon of baking powder, 1/4 teaspoon of baking soda, and 1/4 teaspoon of salt in a separate bowl. Fold into the chocolate mixture using a spatula until just combined.

Pour half of the brownie batter into the prepared baking pan, spread it evenly, drizzle half of the cool raspberry sauce over it, then add the remaining brownie batter on top. Use a knife to gently swirl the raspberry sauce into the brownie batter.

Place the pan in the preheated oven and bake for 30 minutes. A toothpick inserted should come out with a few moist crumbs.

Allow the brownies to cool completely in the pan on a wire rack for about 1 to 2 hours. Lift out using the parchment overhang and slice into squares for serving.