In a medium saucepan over medium heat, combine the raspberries, sugar, and lemon juice. Cook, stirring frequently, until the raspberries break down and the mixture thickens, about 5 minutes. Strain through a fine-mesh sieve to remove seeds and set aside to cool.

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

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

Pour the brownie batter into the prepared baking pan, spreading it evenly. Dollop spoonfuls of the raspberry sauce over the top of the batter. Use a skewer or knife to gently swirl the raspberry sauce into the batter, creating a marbled pattern.

Place the pan in the preheated oven and bake for 45-50 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking.

Allow the brownies to cool completely in the pan on a wire rack. Once cooled, lift them out using the parchment paper and cut into squares. Serve on their own or with a scoop of vanilla ice cream.

Store leftover brownies in an airtight container at room temperature for up to 3 days, or refrigerate for up to a week. For longer storage, wrap them tightly and freeze for up to 2 months.