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, whisking until well combined.
03 - Sift in cocoa powder, flour, salt, and baking powder. Mix gently until just combined; avoid overmixing.
04 - Blend cottage cheese until creamy using a food processor or immersion blender. Add egg yolk, sugar, and vanilla; blend until smooth.
05 - Pour brownie batter into the prepared pan and smooth the top evenly.
06 - Spoon dollops of cottage cheese mixture over the brownie batter. Use a knife or skewer to gently swirl the two batters together for a marbled effect.
07 - Bake for 33-36 minutes, until edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
08 - Cool completely in the pan before slicing into squares.