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. Remove the foil and cut into bars.