01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, allowing 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 until smooth and set aside to cool.
03 - Melt butter and chopped chocolate in a microwave-safe bowl in 30-second intervals, stirring until smooth. Allow to cool slightly.
04 - Whisk sugar, eggs, vanilla, and salt into the chocolate mixture until glossy and thoroughly combined.
05 - Sift flour and cocoa powder over the batter. Fold gently until just incorporated, being careful not to overmix.
06 - Spread brownie batter evenly in the prepared pan. Drop spoonfuls of cooled strawberry puree over the surface. Use a skewer or knife to gently swirl the puree into the batter, creating a marbled pattern.
07 - Bake for 28-32 minutes until a toothpick inserted in the center emerges with a few moist crumbs.
08 - Cool completely in the pan before lifting out using the parchment overhang. Cut into squares and serve.