01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring it covers the bottom and extends up the sides for easy removal.
02 - In a large mixing bowl, whisk melted butter and granulated sugar until completely smooth and no sugar granules remain. Add eggs and vanilla extract, beating until the mixture appears glossy and well combined.
03 - Sift flour, cocoa powder, and salt directly into the wet mixture. Fold gently using a spatula, mixing just until dry ingredients disappear. Avoid overmixing to prevent tough brownies. Fold in chocolate chips if using.
04 - Blend cottage cheese in a food processor or blender until completely smooth with no lumps remaining. Transfer to a separate bowl and whisk in egg yolk, sugar, flour, and vanilla extract until fully incorporated and smooth.
05 - Spread brownie batter evenly into the prepared pan. Drop cottage cheese mixture in spoonfuls across the surface. Run a knife through both mixtures in a figure-eight or swirling pattern to create a marbled effect, being careful not to overmix.
06 - Bake for 32 to 36 minutes until the edges appear set but the center still slightly jiggles. A toothpick inserted in the center should emerge with moist crumbs, not wet batter.
07 - Allow brownies to cool completely in the pan, approximately 2 hours. The cooling process is essential for the texture to set. Lift out using parchment paper edges and cut into 12 even squares.