Preheat the oven to 350 degrees F. Line an 8-inch-square baking pan with foil, pressing it into the corners and up the sides; leave an overhang on 2 sides.
Butter the foil.
Put 4 tablespoons butter and the brown sugar in a large microwave-safe bowl.
Microwave until the butter is melted, about 1 minute.
Stir in the vanilla.
Let cool slightly, then stir in the egg.
Whisk the flour, salt and baking powder in a small bowl.
Stir into the butter mixture.
Stir in the butterscotch chips and nuts and spread the batter in the pan.
Drop dollops of jam on top and swirl with a knife.
Bake until set, 20 to 25 minutes.
Let cool 15 minutes on a rack, then lift the foil and transfer the blondies to the rack to cool completely.