Preheat oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
In a bowl, whisk together melted butter and sugar until smooth. Add eggs and vanilla, and mix to a glossy finish.
Stir in cocoa powder, flour, salt, and baking powder; mix until just combined.
In another bowl, mix cream cheese, sugar, egg yolk, and vanilla extract until smooth.
In a saucepan, combine blackberries, sugar, and lemon juice; heat and mash until slightly thickened, then let cool.
Pour most of the brownie batter into the pan, reserving some for later. Spoon cream cheese and blackberry sauce over the brownie batter, topping with reserved batter.
Use a toothpick to gently swirl the mixtures together.
Bake for 35 to 38 minutes until the center is set.
Allow to cool, then chill for at least one hour before serving.
Notes
Notes
For best flavor, use room temperature cream cheese and fresh, ripe blackberries. Adjust baking time for fudgy texture.