Make the Cake: Preheat oven to 350 degrees F. Grease a 9 x 13-inch baking pan.: The aroma of a preheated oven is the first signal that baking has begun, a warm dry heat that primes the pan for even browning. You should feel the oven stabilizing at 350 degrees F before the batter goes in, which promotes a steady rise and an even set. Greasing the pan helps release later and encourages a golden edge. A common slip here is placing the pan in before the oven is fully up to temperature, which can lead to uneven bake and a denser center; wait until the oven reads true heat. Visually, the batter should begin to set around the edges within the first 12 to 15 minutes, and you will notice the top start to take on a pale gold tone as the oven does its work.
In a medium bowl, stir together the boiling water, raisins and oats and let sit for 20 minutes.: Right away you will notice steam and a sweet raisin perfume as the boiling water hits the dried fruit and oats . This soak plumps the raisins and softens the quick cooking steel cut oats , allowing them to integrate into the batter rather than draw moisture out during baking. After about 20 minutes the raisins should look swollen and glossy, and the oats should be tender but still retain some tooth. If you skip the soak, the raisins will remain dry and chewy in the finished cake, and the oats might stay too firm; soaking prevents both issues. Stir once or twice during the soak to ensure even hydration and to taste a raisin to confirm plumpness.
In a small bowl, whisk together the flour, baking soda, cinnamon, nutmeg and salt; set aside.: As you whisk, the scent of the cinnamon and nutmeg will lift and combine with the neutral smell of the all purpose flour , creating an aromatic base for the batter. Whisking distributes the baking soda and salt so they activate evenly when mixed into the wet ingredients, which is crucial for consistent lift. If the dry ingredients are not well combined, you risk pockets of leavening that cause uneven texture. Visually, the mixture should look uniform with a light dusting of spices through the flour, and you should not see clumps of baking soda .
Using an electric mixer, beat the butter and both sugars together on high speed until light and fluffy, about 5 minutes. Reduce the speed to medium and add the eggs and vanilla extract, beating to combine. Reduce the mixer speed to low and add the oat mixture, then gradually add the flour mixture, beating until just combined. Give the mixture a final stir with a rubber spatula and transfer to the prepared baking pan.: When you start creaming the room temperature butter with the granulated sugar and light brown sugar , listen for a change in resistance and watch the color shift to paler and more aerated; that trapped air is what lightens the final crumb. After about 5 minutes on high, the texture should look fluffy and lifted, and you will see tiny air bubbles in the batter. Adding the eggs at medium speed helps them emulsify, creating a smooth, cohesive batter. When you fold in the soaked oat mixture and then gradually the dry mix, stop mixing as soon as the streaks disappear; over mixing will develop gluten and tighten the crumb. A frequent error is rushing the creaming step or over beating after adding flour which yields a tougher cake. The finished batter should be glossy, moderately thick, and should pour with a slight ribbon into the prepared pan, spreading evenly with a spatula.
Bake until golden brown and a toothpick inserted in the center comes out clean, about 40 to 45 minutes. Place on a wire rack and cool completely before frosting.: As the cake bakes you will notice a deepening aroma of cinnamon and caramelized sugars, and the surface will transition from pale to a warm golden brown. The edges will pull slightly away from the pan, and a toothpick inserted in the center should emerge clean or with a few moist crumbs when it is done; this is the best indicator of doneness. Avoid opening the oven repeatedly which can cause collapse; if the center still looks very jiggly at the time mark, return it to the oven for a few more minutes. Transfer the pan to a wire rack so air can circulate underneath and cool the cake fully, which prevents the frosting from sliding. If you frost warm cake, the frosting will melt and lose structure.
Make the Cream Cheese Frosting: Using an electric mixer, beat the cream cheese and butter on medium-high speed until smooth, about 3 minutes. Reduce the mixer speed to medium-low and gradually add the powdered sugar until it is all incorporated. Add the vanilla extract and salt, increase the mixer speed to medium-high and beat until light and smooth, about 2 minutes. Spread over the cooled cake and serve. Leftovers can be stored at room temperature, covered, for up to 4 days.: When you whip the cold cream cheese with the small amount of room temperature butter , the initial texture will be dense but will quickly soften into a creamy swirl; the sound is a steady, even whir as the beaters move through. Gradual addition of the powdered sugar prevents a gritty texture and helps you control sweetness and consistency. The final frosting should be glossy, fluffy, and easy to spread with a knife, leaving soft peaks as you smooth. One pitfall is using warm cream cheese which makes the frosting runny and unstable; cold cream cheese is intentionally specified for structure. After frosting, the cake should be stored out of direct sunlight and can sit covered at room temperature for a short while, though refrigeration will extend freshness if your kitchen is warm.