01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring paper extends over edges for easy removal.
02 - In a large bowl, whisk melted butter and sugar until smooth and glossy. Add eggs and vanilla, 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, taking care not to overmix. Fold in chocolate chips if desired.
04 - Blend cottage cheese in a food processor or blender until completely smooth, about 30 seconds. Transfer to a separate bowl and whisk in egg yolk, sugar, flour, and vanilla until silky and lump-free.
05 - Spread brownie batter evenly in the prepared pan. Drop spoonfuls of cottage cheese mixture across the surface. Run a knife through both batters in a figure-eight pattern to create a marbled swirl effect.
06 - Bake for 32 to 36 minutes until edges are set but center still slightly soft. 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. Lift out using parchment paper overhang and cut into 12 equal squares with a sharp knife.