Preheat your oven to 350°F (175°C). In a large mixing bowl, beat the melted butter and sugar together until well combined. Add the eggs one at a time, mixing thoroughly after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Fold in the chocolate chips gently.

Mash the raspberries in a small bowl using a fork. Drop spoonfuls of the mashed raspberries over the brownie batter and use a skewer to swirl them into the batter, creating a marbled pattern.

Pour the batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.

Allow the brownies to cool in the pan on a wire rack for at least 30 minutes before slicing. Serve warm or at room temperature, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.

Store any leftover brownies in an airtight container at room temperature for up to 3 days. For longer storage, wrap the brownies tightly and freeze for up to 2 months.