01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring paper extends over sides for easy removal.
02 - Whisk melted butter and granulated sugar in a large bowl until smooth and glossy. Add eggs and vanilla extract, beating until fully incorporated and slightly thickened.
03 - Sift flour, cocoa powder, and salt directly into the wet mixture. Fold gently with a spatula until just combined—do not overmix. Fold in chocolate chips if desired.
04 - Blend cottage cheese in a food processor or blender until completely smooth. Transfer to a bowl and whisk in egg yolk, sugar, flour, and vanilla until no lumps remain.
05 - Spread brownie batter evenly in the prepared pan. Drop spoonfuls of cottage cheese mixture over the surface. Run a knife through both layers in a figure-eight pattern to create swirls.
06 - Bake for 32-36 minutes until edges are set but center still appears slightly underbaked. A toothpick inserted in the center should emerge with moist crumbs, not wet batter.
07 - Allow brownies to cool completely in the pan before lifting out by parchment paper edges. Slice into 12 squares for clean edges.