Preheat oven to 350°F. Line a 9-inch round cake pan with parchment paper or oil and flour pan well; set aside.
Cream butter and sugar until light and fluffy, 7–10 minutes. Meanwhile, combine milk, egg whites and vanilla extract in a small bowl and set aside. Sift together Bob’s Red Mill Super-Fine Cake Flour, baking powder, and salt and set aside.
Add 1/3 of the flour mixture to the butter and briefly mix to combine, followed by ½ of the liquid. Continue to alternately add remaining flour and liquid and mix until combined, taking care not to over-mix the batter.
Transfer batter to the prepared cake pan and spread to evenly fill pan and smooth top. Bake until set and a tested inserted in the center comes out clean, about 25–30 minutes. Let cool thoroughly, at least 1 hour. Meanwhile, prepare frosting.
Frosting
Combine powdered sugar and soft butter and mix on low for 1 minute to combine.
Increase speed to medium and mix until light and fluffy, 3–5 minutes.
Add vanilla extract and milk or cream (if needed to adjust consistency) and mix until combined, about 1 minute more. Set aside frosting and Bob’s Red Mill Shredded Coconut until ready to assemble cake.