1. Position the oven rack in the middle of the oven and preheat to 350°F. Coat three 6-inch cake pans with nonstick cooking spray, line the bottoms with parchment rounds, and spray the parchment.
2. In a medium bowl, whisk together the sifted cake flour, baking powder, and salt.
3. In a blender, combine the milk, egg whites, and 4 cups spinach. Blend until completely smooth.
4. Using an electric mixer, beat the butter on medium-high speed for about 2 minutes, until creamy. Gradually add the sugar and beat for about 3 minutes, until light and fluffy. Scrape down the bowl as needed, then beat in the vanilla.
5. Add the flour mixture in three additions, alternating with the spinach-milk mixture. Begin and end with the flour, mixing briefly on low speed until the batter is smooth.
6. Divide the batter evenly between the prepared cake pans and smooth the tops.
7. Bake for 40–45 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. The cake should be lightly golden around the edges and begin to pull away from the sides of the pans.
8. Cool the cakes in their pans on a wire rack for 8–10 minutes. Remove the cakes, peel away the parchment, and cool completely on the racks.

For the Spinach Buttercream

1. Blend the milk and spinach together until smooth.
2. In a large bowl, beat the butter on medium-high speed for about 2 minutes, until creamy.
3. Gradually add 1 cup of powdered sugar and beat for about 3 minutes, until light and fluffy.
4. Add the remaining powdered sugar, spinach-milk mixture, and vanilla. Beat on high until the frosting is smooth and silky.

To Assemble

1. If needed, carefully trim the tops of the cakes so all three layers are even.
2. Place one cake layer on a serving platter and spread with about 1/3 cup of frosting.
3. Add the second cake layer and spread with another 1/3 cup of frosting.
4. Place the final cake layer on top and use the remaining frosting to cover the top and sides of the cake.
5. Decorate as desired and serve.