01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving overhang for easy removal.
02 - Combine strawberries, sugar, and lemon juice in a small saucepan. Cook over medium heat for 5-7 minutes, stirring occasionally, until strawberries break down and mixture thickens. Mash with fork or blend until smooth. Set aside to cool.
03 - In microwave-safe bowl, melt butter and chocolate in 30-second intervals, stirring until smooth. Let cool slightly.
04 - Whisk sugar, eggs, vanilla, and salt into melted chocolate mixture until glossy and well combined.
05 - Sift in flour and cocoa powder. Fold gently until just incorporated.
06 - Spread brownie batter evenly in prepared pan. Drop spoonfuls of strawberry puree over batter. Use skewer or knife to gently swirl puree into brownie mixture for marbled effect.
07 - Bake for 28-32 minutes, or until toothpick inserted in center comes out with a few moist crumbs.
08 - Cool completely in pan, then lift out and cut into squares.