PREP: Preheat the oven to 325 degrees F (162 degrees C). Line an 8x8-inch baking pan with parchment paper and lightly grease with cooking spray. Set aside.
CAKE: Add the cashews and brown sugar to a large powerful blender (Blend-tec/Vitamix) and blend until cashews are broken down and incorporated with brown sugar, 20-30 seconds. (Don't blend too long-- we don't want to make cashew butter!) Add in the cubed butter and pulse to break it down. Scrape down the sides of the blender and add both eggs, milk, and vanilla extract. Blend until smooth and creamy. Add in the oats, baking powder, baking soda, cinnamon, salt, and cornstarch. Blend until batter is completely smooth, stopping and scraping down edges as needed, about 90 seconds.
BAKE: Pour completely smooth batter into the prepared baking pan, using a spatula to scrape out every bit into the pan. Smooth the top with the spatula and bake for 37-43 minutes or until the cake is lightly browned at the edges and set in the middle of the oven and bake until done--toothpick should come out clean when inserted in the middle.
FROST: Meanwhile, place softened cream cheese in a large bowl. Using a hand mixer, beat the cream cheese with the vanilla and salt until smooth. Slowly add in the powdered sugar, beating between additions, until smooth and creamy and to your desired consistency. Add milk and beat until smooth.
DECORATE: Frost the completely cooled cake with the frosting. If desired, top the frosting with chopped cashews. (See Note 1.)