01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring the paper overhangs the edges for easy removal.
02 - Whisk melted butter and granulated sugar in a large bowl until smooth. 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—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 extract until no lumps remain.
05 - Spread brownie batter evenly in the prepared pan. Drop cottage cheese mixture in 6-8 spoonfuls over the surface. Use a knife to swirl gently through both layers, creating a marbled pattern.
06 - Bake for 32-36 minutes until a toothpick inserted in the center comes out with moist crumbs. The edges should be set but the center slightly soft.
07 - Cool completely in the pan, approximately 1 hour. Lift out using parchment paper edges and cut into 12 equal squares.