01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper.
02 - Whisk melted butter and sugar in a large bowl until smooth. Add eggs and vanilla, mixing until combined. Sift in flour, cocoa powder, and salt. Fold gently until just combined. Stir in chocolate chips if desired.
03 - Blend cottage cheese until completely smooth. Whisk in egg yolk, sugar, flour, and vanilla until fully incorporated.
04 - Spread brownie batter evenly in prepared pan. Dollop cottage cheese mixture on top in spoonfuls. Use a knife to gently swirl for marbled effect.
05 - Bake 32-36 minutes until toothpick inserted in center comes out with moist crumbs.
06 - Cool completely in pan before slicing into squares.