Preheat your oven to 350°F (175°C). Line a 9x9 inch (23x23 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease the parchment paper.

In a large mixing bowl, combine the melted butter and 1 cup (200g) granulated sugar. Whisk until well combined.

Add the 2 large eggs and 1 teaspoon vanilla extract to the butter-sugar mixture. Whisk vigorously for about 1 minute until the mixture is lighter in color and slightly thickened.

Gently fold in the all-purpose flour, unsweetened cocoa powder, and salt with a rubber spatula until just combined. Be careful not to overmix. If using, fold in the semi-sweet chocolate chips.

In a blender or food processor, combine the full-fat cottage cheese, 1/4 cup (50g) granulated sugar, large egg yolk, and 1/2 teaspoon vanilla extract. Blend until completely smooth and creamy, scraping down the sides as needed.

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

Dollop the cottage cheese mixture over the brownie batter. Use a spoon to gently spread it, leaving about a 1/2-inch border from the edges.

Drop the remaining brownie batter in small dollops over the cottage cheese layer. Using a knife or a skewer, gently swirl the brownie and cottage cheese layers together to create a marbled effect. Be careful not to over-swirl.

Bake for 30-35 minutes, or until the edges are set and a toothpick inserted into the brownie (not the cottage cheese swirl) comes out with moist crumbs, but no wet batter. The center of the cottage cheese swirl might still appear slightly jiggly.

Remove the pan from the oven and let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. This is crucial for fudgy, well-set brownies.

Once completely cool, cut into 16 squares and serve. For best results, chill for at least 30 minutes before cutting.