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