Preheat the oven to 350°F (180°C) and line a 9×9-inch baking pan with parchment paper.
In a blender or food processor, blend the strawberries until smooth. Strain through a fine mesh sieve to remove the seeds. Set aside.
9 ounces fresh or frozen strawberries
In a medium bowl, whisk together the flour and salt. In a saucepan over low heat, melt the butter and chocolate together, stirring until smooth.
1¾ cups all-purpose flour, ½ teaspoon salt, 1½ cups butter, 12 ounces chocolate
Remove from heat and whisk in the granulated sugar, brown sugar, eggs, and vanilla extract until fully combined. Add the flour mixture to the wet ingredients and gently fold with a spatula until just combined. Avoid overmixing.
1½ cups granulated sugar, ⅔ cup light brown sugar, 6 large eggs, 1½ teaspoons vanilla extract
Pour the batter into the prepared pan and spread it evenly. Drizzle the strawberry puree on top and swirl with a knife to create a marbled pattern.
Bake for 25–35 minutes, rotating the pan halfway through, until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
Let cool completely in the pan before slicing and serving.