01 - Preheat your 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 sugar in a large bowl until smooth and well combined. Add eggs and vanilla extract, mixing thoroughly until the mixture is glossy and uniform.
03 - Sift flour, cocoa powder, and salt into the wet mixture. Fold gently with a spatula until just combined, being careful not to overmix. Stir in chocolate chips if desired.
04 - Blend cottage cheese in a blender or food processor until completely smooth. Transfer to a separate bowl and whisk in egg yolk, sugar, flour, and vanilla extract until fully incorporated and creamy.
05 - Spread brownie batter evenly into the prepared pan. Drop spoonfuls of the cottage cheese mixture across the surface. Use a knife to gently swirl the two mixtures together, creating a marbled pattern without over-mixing.
06 - Bake for 32-36 minutes until a toothpick inserted in the center comes out with a few moist crumbs. The edges should be set but the center should still appear slightly fudgy.
07 - Allow brownies to cool completely in the pan before lifting out using the parchment paper overhang. Slice into 12 squares and serve.