01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, ensuring the paper extends slightly over the edges for easy removal.
02 - In a large bowl, whisk melted butter and granulated sugar until smooth and well combined. Add eggs and vanilla extract, whisking thoroughly until the mixture is glossy and uniform.
03 - Sift cocoa powder, flour, salt, and baking powder into the wet mixture. Fold gently with a spatula until just combined, being careful not to overmix to maintain the fudgy texture.
04 - Blend cottage cheese in a food processor or with an immersion blender until completely smooth and creamy. Add egg yolk, sugar, and vanilla extract, blending until the mixture is uniform and lump-free.
05 - Spread the brownie batter evenly into the prepared pan, smoothing the surface. Drop spoonfuls of the cottage cheese mixture over the batter, then use a knife or skewer to create a marble pattern by gently swirling through both layers.
06 - Bake for 33-36 minutes until the edges are set and slightly pulling away from the pan sides. A toothpick inserted in the center should emerge with a few moist crumbs, not wet batter.
07 - Allow the brownies to cool completely in the pan before slicing. This ensures clean cuts and allows the flavors to fully develop. For easier slicing, refrigerate for 30 minutes before cutting.