- Preheat the oven to 350°F and grease two 9-inch pans with cake goop or line the bottom of the pan with parchment paper and coat with nonstick spray. 
- Add 4 large egg whites to a stand mixer fitted with a whisk attachment and beat on speed 4 for 2 minutes, then increase to speed 6 until stiff peaks form, about 1 additional minute. Transfer egg whites to a small bowl and set aside. 
- In a separate small bowl or measuring cup, combine the 1 cup buttermilk and 1 teaspoon baking soda and set aside. 
- Add 1⅓ cups sweetened flaked coconut to a food processor and pulse until fine.  
- Add the processed coconut to the now empty stand mixer bowl along with 2 cups cake flour, 2 cups granulated sugar, 2 tablespoons powdered milk, and ¾ teaspoon salt and mix to combine. 
- Add the softened 1 cup unsalted butter and mix until sandy in texture. 
- Scrape down the sides in case any butter is sticking and beat in 5 large egg yolks, 1 tablespoon vanilla bean paste, and 1½ teaspoons coconut extract. 
- Beat in the buttermilk mixture in three additions until fully combined, scraping down the sides as needed. 
- Fold in the stiff egg whites until well combined. 
- Evenly divide the batter between the two pans (about 720g each) and bake for 35 to 40 minutes until a toothpick comes clean.  
- Remove from oven and let cool in the pan for 5 minutes before transferring to a cooling rack until almost completely cool. These cakes are best made the day before frosting. When still a little warm to the touch, wrap them in a double layer of plastic wrap and store them at room temperature overnight - this helps make the cakes nice and moist.