Preheat oven to 350 degrees. Line a 9-inch square baking dish with parchment paper. Set aside.
In a small bowl, whisk together the flour, baking powder and salt.
In a large bowl, whisk together the light brown sugar, dark brown sugar, melted butter, half of the peanut butter, vanilla bean paste and egg.
Add the flour mixture to the butter mixture and mix until combined.
Spread the batter into the prepared baking dish. Place dollops of the remaining peanut butter and the blackberry jam onto the batter. Gently swirl through the batter.
Bake for 25 - 30 minutes or until just set. Remove from the baking dish and cool on a wire rack before cutting into squares.
This recipe is my slightly adapted version of the PB and J Blondies from Southern Cast Iron magazine.