Preparation Steps

• Preheat oven to 350°F. Grease an 8 inch square pan and line with parchment, leaving overhang for easy lifting.
• Make the strawberry swirl: In a small saucepan, combine diced strawberries, 2.0 tbsp sugar, and lemon juice. Cook over medium heat, stirring and mashing, until juicy and bubbling, about 5 to 7 minutes.
• Stir cornstarch with 1.0 tbsp water to make a slurry. Add to the strawberries and simmer 1 to 2 minutes until thick and glossy. Remove from heat and cool to room temperature.
• In a heatproof bowl, melt the butter and chopped chocolate together (microwave in 20 to 30 second bursts or use a double boiler). Stir smooth and let cool 2 minutes.
• Whisk in granulated sugar and brown sugar until combined. Whisk in eggs, one at a time, then vanilla. Whisk vigorously for about 1 minute until glossy.
• Sift in flour, cocoa powder, and salt. Fold gently just until no dry streaks remain. Fold in chocolate chips if using.
• Spread batter evenly into the prepared pan. Spoon cooled strawberry mixture over the top in dollops and swirl with a knife or skewer.
• Bake 28 to 32 minutes, until the top is set with a shiny crust and a toothpick inserted near the center comes out with a few moist crumbs.
• Cool completely in the pan on a rack. Lift out using parchment, slice into 16 squares, and top with sliced fresh strawberries if desired.
• Store airtight at room temperature up to 2 days or refrigerate up to 5 days.