Preheat the oven to 350 degrees F.
In a medium size bowl, add the flour, baking powder, baking soda and salt. Stir together and side aside.
In a large bowl, add the melted butter and brown sugar. Stir together until incorporated.
To the butter mixture, add granulated sugar and stir until incorporated.
To the butter and sugar mixture, add the eggs and vanilla.
Beat with a mixer until smooth.
Add a couple large spoonfuls of the flour mixture to the butter mixture.
Use the slowest setting on a mixer to mix in the flour. Then repeat until all of the flour has been added and mixed in to form the batter.
Add the Biscoff cookie butter to the batter.
Using a spatula, fold it into the batter creating a "swirl" pattern. Set aside for a moment.
Grease the baking pan with butter. Cut a piece of parchment paper so that there is enough excess to hang over the left and right side of the pan (these will be "handles" when removing the pan of blondies from the pan after they have cooled). Press the paper to the inside of the pan and smooth it out over the butter which should assist with keeping the paper attached to the pan.
Add the blondie batter to the pan.
Use a spatula to spread the batter out evenly in the pan.
Bake in a preheated oven until the edges of the blondies start to turn golden brown and pull away from the inside edges of the pan (about 32 to 35 minutes). Remove the pan from the oven on to a baking rack and let the blondies cool completely before cutting them.
After the blondies have cooled completely, cut into the desired serving sizes and serve.