Preheat your oven to 325°F and lightly grease a 9 x 13-inch baking dish.: You will feel the kitchen warm as the oven comes to temperature, and a light sheen of oil on the pan prevents sticking, allowing the cake to release cleanly. Preheating is essential for even rise, because a cold oven will cause uneven baking. A common mistake is under greasing, so take a moment to coat the corners thoroughly.
In a medium bowl, whisk together flour, baking powder, cinnamon, allspice, and salt.: As you whisk, the dry mixture will look uniform and slightly aromatic from the cinnamon and allspice . This step helps distribute leavening and spices so each bite tastes balanced. If you skip thorough mixing, you can get pockets of spice or uneven rise, so whisk until you see consistent color.
To a large bowl, add eggs and whisk until lightly beaten. Add sugar and vegetable oil and beat until well-mixed. Add flour mixture and use a wooden spoon to stir until just mixed.: The initial whisking of the eggs introduces air, and when the sugar and vegetable oil join, the batter will shine slightly and feel fluid. When you fold in the dry mix with a wooden spoon, aim for a smooth batter without streaks of flour. Overmixing at this stage can develop too much gluten, producing a tougher cake, so stop when no dry streaks remain.
Add carrots and stir until evenly distributed, scraping the sides and bottom of the bowl with a rubber spatula to make sure all of the batter is mixed.: As you fold in the grated carrots , you will see orange flecks throughout the batter and feel a slight increase in thickness. Scraping the bowl ensures no pockets of flour hide on the sides, and it promotes an even crumb. A common error is adding the carrots too early or mixing them aggressively, which can make the batter gummy, so fold gently.
Pour batter into prepared baking dish and smooth the top. Bake for about 50 minutes to 1 hour, until the center of the cake looks set and a toothpick or cake tester inserted into the center of the cake comes out clean.: Pouring the batter will create a glossy surface that you can smooth with a spatula. As the cake bakes, you will notice a warm, spiced aroma filling the kitchen and the edges pulling slightly away from the pan. Watch for a light golden color on top, and test with a toothpick; if it comes out clean, the cake is done. A frequent mistake is underbaking; if the center is wobbly, give it another 5 to 10 minutes.
Cool cake in the pan until completely cool.: Cooling is when the crumb firms up and is essential before frosting to prevent melting. The cake will stop releasing steam and will feel room temperature to the touch when ready. Rushing to frost a warm cake causes the frosting to slide; patience here is rewarded.
In a large bowl, beat cream cheese, butter, and vanilla until well-combined and smooth. Slowly add sifted powdered sugar and beat until all the sugar is incorporated and frosting is smooth and creamy.: When you beat the cream cheese and salted butter , the mixture will go from lumpy to silky, emitting a rich dairy aroma. Adding the sifted powdered sugar gradually prevents a gritty texture and helps you control sweetness. If the frosting seems too soft, chill briefly and rewhip; a common slip is overbeating after adding the sugar, which can introduce too much air.
Spread frosting over the top of the cake. Chill before serving if desired, and enjoy!: Spreading the frosting should feel creamy under your spatula, and the cake will look polished and inviting. Chilling firms the frosting for cleaner slices and lets flavors meld. Avoid serving immediately from the fridge without letting the cake sit for a few minutes, because ice cold frosting can mask flavor nuances.