I start by heating the oven to 350°F (175°C). I line my baking pan with parchment paper, leaving some edges hanging over—it makes lifting out the brownies so much easier later.
In a small bowl, I mash the blackberries with sugar and lemon juice. Sometimes I strain out the seeds for a smoother layer, but it’s totally optional depending on your texture preference.
I whisk the melted butter with both sugars until glossy, then mix in the eggs and vanilla. After that, I sift in the cocoa, flour, baking powder, and salt. I fold everything gently so the brownies stay fudgy instead of cakey.
In another bowl, I beat the cream cheese, sugar, egg yolk, and vanilla until smooth. I avoid over-mixing here, because that can make the cheesecake layer runny.
I pour half of the brownie batter into the pan, then add dollops of the cheesecake mixture and blackberry puree. I repeat with the remaining batter, then swirl everything together with a skewer to get that gorgeous marbled effect.
The pan goes into the oven for about 40–45 minutes. I check with a toothpick—if it comes out with moist crumbs (not wet batter), they’re ready.
Once baked, I let them cool for 15 minutes in the pan, then transfer to a wire rack. For perfect slices, I chill them in the fridge for an hour before cutting into squares.