Prepare the Cake Layers

1. Preheat your oven to 350°F (175°C). Grease and line three 8-inch cake pans with parchment paper.
2. In a bowl, sift together cake flour, baking powder, and salt. Set aside.
3. In a large bowl, cream butter and sugar until light and fluffy (about 3 minutes).
4. Add egg whites one at a time, beating well after each addition.
5. Mix in vanilla extract and melted white chocolate.
6. Alternately add dry ingredients and milk/sour cream, beginning and ending with dry ingredients.
7. Divide batter evenly between the pans and bake for 22-25 minutes, or until a toothpick inserted comes out clean.
8. Let cakes cool in the pans for 10 minutes, then transfer to a wire rack.

Make the Coconut-Pecan Filling

1. In a saucepan over medium heat, whisk together evaporated milk, sugar, and egg yolks.
2. Stir in butter and cook until thickened (about 10 minutes).
3. Remove from heat and stir in coconut, pecans, and vanilla extract. Let cool completely.

Whip Up the White Chocolate Buttercream

1. In a large bowl, beat butter until creamy.
2. Gradually add powdered sugar, beating until smooth.
3. Mix in melted white chocolate, heavy cream, vanilla, and salt until fluffy.

Assemble the Cake

1. Place one cake layer on a plate. Spread a generous layer of coconut-pecan filling on top.
2. Repeat with the second layer, then top with the third.
3. Cover the cake with white chocolate buttercream, smoothing the sides with an offset spatula.
4. Garnish with extra coconut and pecans if desired.