Preheat the oven to 325°F, grease two 9-inch round cake pans with cake goop, or use non-stick spray and line the bottom with parchment paper, and set aside. Add 3½ cups cake flour, 2 cups granulated sugar, 2 tablespoons powdered milk, 4 teaspoons baking powder, and 1 teaspoon salt to a stand mixer bowl fitted with a paddle attachment and mix for 1 minute.
Add 1 cup unsalted butter and mix at medium-low speed until the mixture becomes sandy in texture.
Add the 2 large eggs and 4 large egg whites one at a time, mixing after each addition. Scrape down the sides of the bowl.
Add 1 tablespoon vanilla bean paste, then 1 cup whole milk in three parts, mixing for 1 minute after each addition and scraping down the bowl. Mixing until the batter is smooth.
Divide the batter between the two prepared pans. (I usually get about 765 to 780g per pan).
Smooth out the top of the batter and bake for 35 to 45 minutes (see notes) until the edges of the cake begin to pull away from the pan and the top springs bake when touched. A cake tester should also come back clean from the center, and the cake should read an internal temperature of 200°F. If using 8-inch pans, the cakes will likely need 5 to 7 additional minutes.
Immediately after removing from the oven, use a pastry brush to the top of the cakes with some of the ⅓ cup vanilla simple syrup. Allow the cakes to cool in the pans for 10 minutes.
After 10 minutes, flip them onto a wire rack and brush the bottoms and sides of the cakes with the remaining simple syrup and allow them to cool completely before frosting. About 1 hour.