Preheat the oven to 350 degrees Fahrenheit. Spray a 10 x 15 x 1 baking sheet (jelly roll pan) with nonstick cooking spray and set aside.: The air in the oven should smell faintly warm and slightly dry when it reaches 350 degrees Fahrenheit , which creates the environment for an even rise. I always let the oven fully preheat because putting batter into an oven that is still warming can cause uneven bake and a denser center. A light coating of nonstick spray prevents the bars from clinging to the pan, and you should run your hand gently over the pan surface to ensure full coverage. Common mistake to avoid, don’t skip preheating, or you may end up with a soggy middle and overbrowned edges.
Whisk together 2 1/2 cups flour, 2 teaspoons ground cinnamon, 1/2 teaspoon ground nutmeg, 1/2 teaspoon ground ginger, 1/4 teaspoon ground cloves, 1/2 teaspoon baking soda, 1 1/2 teaspoons baking powder, and 1 teaspoon salt in a medium bowl and set aside.: You should see a uniform light brown color from the spice blend and no clumps of any single ingredient, which means the leaveners and spices are evenly distributed. Whisking aerates the dry mix slightly, helping the batter incorporate smoothly later. If spices are clumpy, press them through the whisk so they evenly flavor the cake. One common error is under mixing the dry ingredients, which can leave pockets of baking soda that create odd tasting spots, so take your time here.
Add 1 cup vegetable oil, 1 cup packed light brown sugar, and 1 cup granulated sugar to a large bowl and mix until combined. Add 3 large eggs and 2 teaspoons vanilla extract and mix until combined.: When you beat these together, the mixture should look glossy and cohesive, and you will notice a rich aroma from the brown sugar and vanilla . The oil keeps the batter tender while the eggs provide structure. If the mixture seems separated, continue mixing briefly until it looks homogeneous. Avoid overbeating which can incorporate too much air and cause the bars to crack while baking.
Add the flour mixture to the sugar mixture and mix until just combined.: When you fold the dry mix in, stop as soon as you no longer see streaks of flour; small streaks are okay because over mixing develops gluten and will make the bars tough. The batter should be cohesive and slightly thick, not runny. You may hear the quiet sound of batter scraping the bowl, and the batter should hold together on the spatula. A frequent mistake is vigorous mixing here, which results in a chewier texture, so be gentle.
Fold in 2 cups shredded carrots until evenly dispersed in the batter. Pour the batter into the prepared pan and spread it evenly.: As you fold in the carrot , you should see little orange ribbons throughout the batter, and the mixture will gain a bit more moisture and texture. Use a flexible spatula to sweep the bowl clean and make sure there are no pockets of batter left behind. When spreading in the pan, aim for an even surface so the bars bake uniformly; a lumpy surface leads to inconsistent bake. One thing to avoid is adding too much shredded carrot , which can weigh down the batter and make it underbaked in the center.
Bake for 28-34 minutes, until a toothpick comes out clean from the center. Remove the bars from the oven and let them cool completely.: During baking you will smell warm spices and a toasty note from the sugars caramelizing at the edges. The top should be set and lightly golden around the edges, and a toothpick inserted in the center should come out clean or with a few moist crumbs. Allowing the bars to cool fully is critical because frosting a warm pan can cause the frosting to melt and slide off. A common pitfall is cutting too soon, which yields greasy slices and a messy look, so be patient.
Add 1/2 cup room temperature unsalted butter and 6 ounces softened cream cheese to a large bowl and beat on medium-high speed with a hand mixer or stand mixer fitted with the paddle attachment until smooth. Scrape down the sides and bottom of the bowl to ensure it is all incorporated evenly.: The butter and cream cheese should whip into a silky, pale mixture, and you should not see lumps. Scraping down the bowl ensures no pockets of unincorporated dairy remain, which would make the frosting uneven. The sound from the mixer will change to a smoother, lower pitch when fully combined. Avoid using cold cream cheese , which will lump and make the frosting grainy.
Add 3 cups powdered sugar, 2 teaspoons vanilla extract, and 1 large pinch salt to the bowl and beat until smooth.: As the powdered sugar folds in, the frosting will thicken and become glossy. The vanilla brightens the flavor, and the pinch of salt balances the sweetness. If the frosting is too stiff, a splash of milk or a little extra mixing can smooth it, while overbeating can make it too soft, so adjust carefully. A common issue is adding the sugar too fast, which can cause a dusty cloud in your mixer, so add it gradually.
Spread the frosting evenly over the top of the cooled carrot cake. Cover and store in the refrigerator until you are ready to slice it into bars and serve.: The cooled bars will accept the frosting cleanly, and spreading should leave a smooth, even layer that sets slightly in the refrigerator. The frosting should feel cool and creamy to the touch and will firm up after chilling for at least 30 minutes. Store the frosted bars covered in the fridge to keep them fresh; leaving them out too long may soften the frosting and change texture. Avoid slicing right after frosting, as the edges will smear and you will lose the neat look you worked for.