01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring excess paper overhangs 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.
03 - Sift flour, cocoa powder, and salt 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 separate bowl and whisk in egg yolk, sugar, flour, and vanilla extract until silky and uniform.
05 - Spread brownie batter evenly into the prepared pan. Drop cottage cheese mixture in spoonfuls across the surface. Run a knife through both batters in a figure-eight pattern to create a marbled swirl effect.
06 - Bake for 32-36 minutes until a toothpick inserted into the center emerges with moist crumbs but not wet batter. The edges should be set while the center remains slightly fudgy.
07 - Allow brownies to cool completely in the pan, approximately 2 hours. Lift out using parchment paper overhang and cut into 12 equal squares.