These crowd-pleasing bars make a wholesome option for bake sales and lunch boxes.
Adjust the oven rack so that it's in the center and heat the oven to 350°. Coat a 9- by 13-inch baking dish with cooking spray.
Peel and trim the carrots. Use a box grater to finely shred each carrot on a sheet of waxed paper (you should end up with about 3 cups). Tip: To avoid scraped knuckles, stop grating each carrot when there's still about an inch of it left.
In a medium bowl, whisk together the flour, baking soda, cinnamon, and salt. Set the mixture aside.
With a hand or stand mixer set on medium speed, blend the butter and both sugars until pale and fluffy, about 3 minutes. Add the eggs and the vanilla and continue beating on medium speed until well blended. Reduce the mixer's speed to low and add the carrots (use the waxed paper to dump them into the bowl). Mix in the butterscotch chips and, if desired, the walnuts.
Use a wooden spoon to blend the flour mixture into the wet ingredients, scraping down the sides of the bowl as you work. Pour the batter into the prepared dish and use a large spoon or spatula to spread it evenly.
Bake the batter, rotating the pan halfway through, until it has risen slightly and the top has formed a thin, brittle crust, about 30 to 35 minutes. Be careful not to overbake. To test for doneness, insert toothpicks 1 inch from the edge, 2 inches from the edge, and in the center. The first toothpick should come out clean, the second with a little batter, the third with a little more batter.
Place the pan on a wire rack and let it cool completely. Run a knife around the rim of the pan, then divide the contents into 24 bars.