In a small saucepan, combine blackberries, sugar, and lemon juice over medium heat. Allow the mixture to simmer gently, mashing the berries lightly until thick and jammy, about 5–7 minutes. If your blackberries are particularly juicy, add cornstarch to thicken the mixture. Set aside to cool.

Preheat your oven to 350°F (175°C). Line an 8x8” or 9x9” baking pan with parchment paper for easy removal.

In a mixing bowl, whisk together melted butter and granulated sugar until well combined. Add eggs and vanilla extract, stirring until the mixture is smooth.

Sift in cocoa powder, flour, and salt. Stir until just combined, and then fold in chocolate chips for that extra layer of chocolatey goodness. Pour the brownie batter into the prepared pan, smoothing the top.

In a separate bowl, beat the softened cream cheese, sugar, egg, and vanilla extract together until completely smooth and creamy.

Gently spread the cheesecake mixture over the brownie batter, being careful not to disturb the brownie layer underneath.

Using a spoon, dollop the cooled blackberry mixture over the cheesecake layer. Then, use a knife or skewer to swirl gently, creating a marbled effect.

Bake for 35–40 minutes or until the center is just set, allowing a slight jiggle in the middle. Remove from the oven and cool completely in the pan. Chill in the refrigerator for at least 1 hour before slicing to ensure clean layers.