01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring overhang on sides for easy removal.
02 - In a large bowl, whisk melted butter and sugar until smooth and glossy. Add eggs and vanilla extract, beating until fully incorporated and slightly thickened.
03 - Sift flour, cocoa powder, and salt into the wet mixture. Fold gently with a spatula until just combined, being careful not to overmix. Stir in chocolate chips if desired.
04 - Blend cottage cheese in a blender or food processor until completely smooth, about 30 seconds. Transfer to a bowl and whisk in egg yolk, sugar, flour, and vanilla extract until silky and uniform.
05 - Spread brownie batter evenly in the prepared pan. Drop cottage cheese mixture in spoonfuls across the surface. Run a knife through both layers in a swirling motion to create a marbled pattern.
06 - Bake for 32-36 minutes until edges are set but center still appears slightly soft. A toothpick inserted in the center should emerge with a few moist crumbs, not wet batter.
07 - Allow brownies to cool completely in the pan, about 2 hours. Lift out using parchment paper overhang and cut into 12 equal squares.