Step 1

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 for now.

Step 2

CAKE: Add the shelled pistachios and brown sugar to a large powerful blender (Blend-tec/Vitamix) and blend until pistachios are broken down and incorporated with brown sugar-- 20-30 seconds (don't blend too long we don't want to make pistachio butter!). Add in the cubed butter and pulse to break down butter. Scrape sides of blender and add in both eggs, milk, and vanilla extract. Blend until smooth and creamy. Add in the oats, baking powder, baking soda, salt, and cornstarch. Blend until the batter is completely smooth, stopping and scraping down edges as needed, about 90 seconds.

Step 3

BAKE: Pour completely smooth batter into prepared baking pan using a spatula to scrape out every bit. Smooth the top with the spatula and bake for 37-43 minutes or until cake is lightly browned at edges and set in the middle (toothpick when inserted in the center should come out clean).

Step 4

FROST: Meanwhile, place softened cream cheese in a large bowl. (Give it ample time to set out so your frosting isn't chunky.) Using a hand mixer, beat the cream cheese with the vanilla and salt until smooth. Slowly add in the powdered sugar (beating in between additions) until smooth and creamy and to the desired consistency. Add milk and beat until smooth.

Step 5

DECORATE: Frost the completely cooled cake evenly with the frosting. If desired, top the frosting with chopped pistachios.