Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease the parchment paper with cooking spray or butter.

In a small saucepan, combine the diced strawberries, 2 tablespoons granulated sugar, lemon juice, and cornstarch. Cook over medium heat, stirring occasionally, until the strawberries soften and the mixture thickens slightly, about 5-7 minutes. It should look like a chunky, glossy sauce. Remove from heat and let cool slightly while you make the brownie batter.

In a microwave-safe bowl or small saucepan, melt the butter. Pour the melted butter into a large mixing bowl. Whisk in the 1.5 cups granulated sugar until well combined. Add the room temperature eggs one at a time, whisking well after each addition until the mixture is light and fluffy. Stir in the vanilla extract.

In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. If using, stir in the semisweet chocolate chips. This helps prevent them from clumping in the batter.

Gradually add the dry ingredient mixture to the wet ingredients, folding with a rubber spatula until just combined. Be careful not to overmix, as this can lead to dry, cake-like brownies instead of fudgy ones.

Pour about half of the brownie batter into the prepared baking pan and spread it evenly. Dollop spoonfuls of the strawberry swirl mixture over the batter. Gently drag a knife or skewer through the batter to create a swirled pattern. Carefully spread the remaining brownie batter over the strawberry layer, then dollop the remaining strawberry mixture on top and swirl again.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should look set. Let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang and cutting into squares. Cooling completely is key for clean cuts!