Line an 8-inch baking pan with parchment paper or aluminum foil, allowing extra material to extend over the sides so the brownies can be lifted out easily after cooling.

Lightly mist the lined pan with cooking spray to help prevent the brownies from adhering to the surface.

Place the raspberries, sugar, and a small splash of water into a saucepan. Warm the mixture over medium heat, stirring until the sugar dissolves and the fruit begins to break down into a sauce.

Crush the raspberries with a spoon, or blend them with an immersion blender for a smoother consistency. Let the sauce boil briefly for a few minutes, then remove it from the heat.

Pass the raspberry mixture through a strainer to separate out the seeds. Reserve the smooth sauce and set it aside.

Preheat the oven to 350°F (175°C).

In a large saucepan, melt the unsalted butter together with the semi-sweet chocolate chips over low heat. Stir until the mixture becomes glossy, uniform, and completely smooth.

Remove the chocolate mixture from the heat and allow it to cool. The pan may be placed in the refrigerator briefly to help it cool more quickly.

In a separate mixing bowl, whisk the eggs, granulated sugar, and vanilla extract until fully combined.

Gradually pour the egg mixture into the cooled chocolate mixture, stirring constantly as it is added so the eggs blend evenly without curdling.

Add the flour, salt, baking powder, and baking soda to the chocolate mixture. Stir until the dry ingredients are fully incorporated and the batter is smooth.

Transfer the brownie batter to the prepared baking pan, spreading it into an even layer.

Drop small spoonfuls of the raspberry sauce across the surface of the batter. Drag a knife gently through the sauce and top layer of batter to form a marbled swirl pattern.

Bake for 30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.

Let the brownies cool completely in the pan. Once cooled, lift them out using the parchment or foil overhang, then slice and serve.