Combine hulled and quartered strawberries, 2 tablespoons granulated sugar, and lemon juice in a medium saucepan. Cook over medium heat, stirring occasionally, until the strawberries soften and release their juices, about 5-7 minutes. Mash gently with a fork or potato masher to create a chunky puree.

Remove from heat and let cool completely. This can be done while preparing the brownie batter.

Preheat your oven to 350°F (175°C). Grease and line a 9x9 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.

In a large mixing bowl, whisk together the melted butter, 1 cup granulated sugar, and light brown sugar until well combined and smooth.

Add the eggs one at a time, whisking vigorously after each addition until fully incorporated and the mixture is glossy. Stir in the vanilla extract.

In a separate medium bowl, whisk together the cocoa powder, all-purpose flour, and salt. (Optional: Sift the dry ingredients for a smoother batter).

Gradually add the dry ingredients to the wet ingredients, folding with a rubber spatula until just combined. Be careful not to overmix. Fold in the chocolate chips, if using.

Pour about two-thirds of the brownie batter into the prepared baking pan and spread evenly.

Dollop spoonfuls of the cooled strawberry swirl over the batter. Cover with the remaining brownie batter, spreading gently.

Using a knife or skewer, gently swirl the strawberry puree through the brownie batter to create a marble effect. Do not overmix the swirl.

Bake for 28-32 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should be set, but the center slightly fudgy.

Remove from oven and let cool completely in the pan on a wire rack before lifting out with the parchment paper and cutting into squares. Cooling completely is crucial for fudgy, neat brownies.