Preheat your oven to 350°F (175°C) and grease and flour two 9-inch round cake pans.
In a large bowl, combine the white cake mix, buttermilk, vegetable oil, egg whites, and vanilla extract. Beat on medium speed until smooth.
Divide the batter into the prepared pans and bake for 25-30 minutes until a toothpick comes out clean. Cool in pans for 10 minutes, then transfer to wire racks.
For the frosting, melt butter in a saucepan, then add sugar, evaporated milk, egg yolks, and vanilla. Stir until it thickens (10-12 minutes).
Remove from heat, stir in coconut and pecans, and let cool slightly.
Assemble the cake by placing one layer on a platter, frosting it, adding the second layer, and frosting the top and sides.
Slice and serve.