Position a rack in the center of the oven and preheat the oven to 325 degrees F. Grease a 9x13-inch baking pan or heatproof glass dish. Line the pan with parchment paper, leaving about 1" of paper overhanging the two long sides.
In a bowl, whisk together the flour and salt to blend. Set aside.
In the bowl of an electric mixer set on medium speed, cream the butter, granulated sugar, and brown sugar. Beat in the eggs, one at a time, blending thoroughly and stopping to scrape down the sides of the bowl after each addition. Mix in the vanilla.
Stir in the flour mixture until well combined. Stir in 1/2 cup of the chocolate pieces, 1/2 cup of the butterscotch pieces, and 1/2 cup of the walnuts. Spread the batter evenly in the prepared pan with the back of a spoon or a rubber spatula (SEE RECIPE NOTES BELOW). Sprinkle the remaining chocolate pieces, butterscotch pieces and walnuts evenly over the top.
Bake for 30 minutes, or until a wooden skewer inserted near the center comes out clean. Set the pan on a wire rack to cool for 15 minutes. Grasp the edges of the parchment and carefully lift the blondies out of the pan. Cut into bars.
*The batter is very thick and a little tough to spread onto the parchment (the parchment wanted to move around). Once you get it all spread out onto the bottom of the pan, it bakes up beautifully.
*When you're doing the spreading of the batter, I'd suggest spraying your spoon or rubber spatula with a little nonstick spray first- it'll be easier to spread and the batter won't stick to the spoon.