Prep Mode. Preheat your oven to 350°F (175°C). Line your 8x8 inch pan with parchment paper, leaving some overhang on the sides for easy brownie removal later. Trust me, future you will appreciate this step.
Berry Business. In a small saucepan, combine raspberries, sugar, and lemon juice. Heat over medium-low, crushing the berries with a wooden spoon as they warm. Once bubbling, stir in the cornstarch slurry and cook for 1-2 minutes until thickened. Remove from heat, strain through a fine-mesh sieve to remove seeds (unless you're into that texture), and set aside to cool.
Chocolate Magic. In a medium saucepan (or microwave-safe bowl), melt butter and chopped chocolate together on low heat, stirring frequently. Once smooth, remove from heat and let cool slightly. We want melted chocolate, not scrambled eggs in the next step.
Building the Batter. Whisk sugar into the chocolate mixture until combined. Add eggs one at a time, whisking well after each. Stir in vanilla. Switch to your spatula and gently fold in flour, cocoa powder, and salt just until combined – do not overmix! Fold in chocolate chips if using. (Spoiler: you should be using them.)
The Artistry Begins. Pour the brownie batter into your prepared pan. Dollop spoonfuls of the raspberry mixture on top. Use a toothpick or skewer to swirl the raspberry through the brownie batter with figure-eight or S-shaped motions. Don't overdo it – we want distinct swirls, not a fully mixed pink brownie situation.
Bake to Perfection. Bake for 25-30 minutes. The center should be just set but still slightly gooey – a toothpick inserted should come out with a few moist crumbs, not wet batter. Remember: slightly underbaked is better than overbaked when it comes to brownies!
The Hardest Part. Let the brownies cool completely in the pan. I know, I know – this requires superhuman patience. But cutting into hot brownies gives you mess, not squares. For the cleanest cuts, refrigerate for an hour after cooling, then use a sharp knife wiped clean between cuts.