01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, allowing overhang for easy removal.
02 - Combine raspberries, sugar, and lemon juice in small saucepan. Cook over medium heat, stirring occasionally, until raspberries break down and mixture thickens, about 5 minutes. Press through fine-mesh sieve to remove seeds. Set aside to cool.
03 - Set heatproof bowl over pan of simmering water. Melt butter and chocolate together, stirring until smooth. Remove from heat and cool slightly.
04 - Whisk sugar into chocolate mixture. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
05 - Sift in flour, cocoa powder, and salt. Gently fold until just combined. Avoid overmixing.
06 - Pour batter into prepared pan and smooth top. Drop spoonfuls of raspberry puree over batter. Use knife or skewer to gently swirl raspberry into brownie mix, creating marbled pattern.
07 - Bake for 30-35 minutes until toothpick inserted in center comes out with few moist crumbs.
08 - Cool completely in pan. Lift out using parchment overhang and cut into squares.