Make the Cake: Preheat oven to 350 degrees F. Grease a 9x13-inch baking pan.
In a medium bowl, stir together the boiling water, raisins and oats and let sit for 20 minutes.
In a small bowl, whisk together the flour, baking soda, cinnamon, nutmeg and salt; set aside.
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.
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.
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.