Preheat oven to 350° and spray a 9 x 13 baking dish with cooking spray. In a large bowl, combine sugar, eggs and oil. Stir in flour and cinnamon (batter will be thick). Stir in apples and nuts. Spread into prepared pan and sprinkle with butterscotch chips.: When the oven reaches 350° you want even ambient heat so the bars bake uniformly, producing gently browned edges and a set center. I always let my oven fully preheat for at least 10 minutes and position the rack in the center to avoid overbrowning. The smell of the kitchen will be neutral at this stage, but you may notice a faint warmth when you open the door. A common mistake is not preheating long enough, which can make the center take much longer to set, so resist the urge to slide the pan in too early.
Bake at 350° for 35-40 minutes or until golden and a toothpick inserted near the center comes out clean. Cool before cutting.: The mixture should become smooth and slightly glossy, with the sugar integrating into the eggs and canola oil . Whisking for about one to two minutes by hand brings a uniform pale color and ensures the sugar begins dissolving. You may notice a slight syrupy sheen, which indicates proper blending. If the eggs are cold the oil can form small pockets, so allow them to warm briefly on the counter first. Avoid overbeating which can introduce excess air and alter texture.
Stir in flour and cinnamon (batter will be thick): After adding the self rising flour and ground cinnamon , the batter should shift from loose to a thick, scoopable consistency, not runny. Use a spatula to fold gently until no streaks of flour remain, watching for a uniform speckling of spice. The sound will be muted, a soft scraping as the batter gathers. Overmixing here develops gluten and can toughen the bars, so stop when combined. If the batter feels overly dry, check your measuring; too much flour is a common error.
Stir in apples and nuts: Once the batter is thick, fold in the diced apples and chopped walnuts until they are evenly distributed. You should see juicy apple pieces throughout and small nut fragments creating texture. The apples will release moisture during baking, so the batter should feel slightly studded and not watery. A frequent mistake is adding wet fruit without draining, which can thin the batter and affect bake time, so make sure apple pieces are not overly soaked.
Spread into prepared pan and sprinkle with butterscotch chips: Press the thick batter into the greased 9 x 13 pan so the surface is level, then scatter the butterscotch chips evenly. The chips will melt on top and create glossy, sweet pockets. When you run a spatula across the surface you should feel a firm but pliable texture. If batter resists spreading, wet the spatula lightly or dip it in warm water to smooth without compressing. Avoid packing the batter too tightly, which can yield denser bars.
Bake at 350° for 35 to 40 minutes or until golden and a toothpick inserted near the center comes out clean: As the bars bake you will notice a warm, spiced aroma rise from the oven and the edges turning a light golden brown, signaling progress. The center should be set with a slight spring when touched; a clean toothpick near the middle confirms doneness. Listen for silence, there should be no sizzling and only gentle oven hum. A common pitfall is relying solely on color when darker pans bake faster, so always use the toothpick test to avoid underbaking.
Cool before cutting: After removing from the oven, let the pan cool on a wire rack until the top firms and the interior finishes setting, which helps the squares slice cleanly. The residual heat finishes any tiny pockets of unset batter and allows the butterscotch chips to resolidify slightly. You may notice the aroma deepen as it cools. Cutting too soon causes crumbly edges and sticky slices, so exercise patience for the best presentation.