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 - In a large bowl, whisk melted butter and granulated sugar until smooth and well combined. Add eggs and vanilla extract, whisking until the mixture is glossy and uniform.
03 - Sift cocoa powder, flour, salt, and baking powder directly into the wet mixture. Fold gently with a spatula until just combined, being careful not to overmix.
04 - Blend cottage cheese in a food processor or with an immersion blender until completely smooth. Add egg yolk, sugar, and vanilla extract; blend until the mixture is creamy and uniform.
05 - Pour the brownie batter into the prepared pan, spreading evenly with a spatula. Spoon dollops of the cottage cheese mixture over the surface.
06 - Use a knife or skewer to gently swirl the cottage cheese mixture into the brownie batter, creating a marbled pattern. Avoid over-swirling to maintain distinct layers.
07 - Bake for 33 to 36 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs. The center should still be slightly fudgy.
08 - Allow the brownies to cool completely in the pan before lifting out using the parchment paper overhang. Slice into 12 squares and serve at room temperature or chilled.