Preparation

For the Cake:

1. Prepare the Oven: First, preheat your oven to 350°F (175°C). Grease three 9-inch round cake pans and line the bottoms with parchment paper for easy removal.
2. Mix Dry Ingredients: Next, in a medium bowl, sift together the flour, baking powder, baking soda, and salt.
3. Cream Butter and Sugar: Meanwhile, in a large mixing bowl, beat the butter and sugar until light and fluffy. This should take about 3-4 minutes on medium speed.
4. Add Eggs and Vanilla: Gradually add eggs, one at a time, followed by vanilla extract, ensuring each egg is fully incorporated before adding the next.
5. Incorporate Melted White Chocolate: At this point, add the melted white chocolate to the batter, mixing well until smooth.
6. Alternate Flour and Buttermilk: Slowly alternate between adding the flour mixture and buttermilk, starting and ending with the flour. Mix until just combined, taking care not to over-mix.
7. Divide and Bake: Then, divide the batter evenly among the prepared pans. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.
8. Cool the Cakes: Once baked, let the cakes cool in the pans for 10 minutes, and then transfer them to a wire rack to cool completely.

For the Coconut-Pecan Filling:

1. Heat Ingredients: In a saucepan over medium heat, combine the evaporated milk, sugar, egg yolks, and butter, stirring constantly. Cook until the mixture thickens, about 10-12 minutes.
2. Add Coconut and Pecans: Afterward, remove the pan from heat and stir in the vanilla, shredded coconut, and pecans.
3. Cool Completely: Allow the filling to cool to room temperature; it will thicken further as it cools.

For the White Chocolate Frosting:

1. Cream Butter and Melted Chocolate: Start by beating the softened butter until creamy. Add the melted white chocolate and continue mixing until smooth.
2. Add Powdered Sugar and Vanilla: Gradually incorporate the powdered sugar, one cup at a time, beating well after each addition. Then, add vanilla extract and mix thoroughly.
3. Adjust Consistency: To achieve the desired consistency, add heavy cream one tablespoon at a time until spreadable.

Assembly:

1. Layer the Cake: Begin by placing the first layer of cake on a serving plate. Spread a generous amount of the coconut-pecan filling on top, and then place the second layer over it.
2. Top Layer and Frosting: Now, add the final layer on top and cover the cake with the white chocolate frosting, smoothing it over the top and sides.
3. Decorate (Optional): Finally, for a decorative touch, sprinkle extra coconut and chopped pecans on top of the cake.