Make Coconut Custard: In a medium saucepan over medium heat, combine whole milk and canned unsweetened coconut milk and gently simmer. Whisk together the egg yolks, granulated sugar, and cornstarch in a bowl. Slowly add the hot milk mixture, stirring continuously until thickened, about 5-7 minutes. Remove from heat, pour into a bowl, cover with plastic wrap, and cool completely in the refrigerator.

Prepare the Cake Batter: Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a separate bowl, mix together the gluten-free all-purpose flour blend, baking powder, baking soda, and salt. In a stand mixer, cream unsalted butter and sugar until light and fluffy. Incorporate the egg whites and vanilla extract and mix well. Gradually alternate adding the dry ingredients and milk mixture, mixing until combined.

Bake the Cake: Divide the batter evenly between the prepared cake pans and smooth the tops. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the centers comes out clean and the tops are golden brown. Once baked, let the cakes cool in the pans for about 10 minutes before carefully inverting them onto wire racks to cool completely.

Toast Coconut: Spread sweetened flaked coconut on a baking sheet in a single layer. Place in the oven at 350°F (175°C) for about 5-7 minutes, stirring halfway through, until golden brown and fragrant. Set aside to cool.

Make Simple Syrup: In a small saucepan, combine equal parts water and granulated sugar (about 1 cup each) and bring to a boil over medium heat. Stir until the sugar has dissolved, then remove from heat and let it cool.

Prepare Buttercream: In a mixing bowl, beat softened unsalted butter until creamy. Gradually add powdered sugar while mixing on low speed, then increase to medium until light and fluffy. Incorporate the chilled coconut custard and a pinch of salt, mixing until fully combined and smooth.

Assemble Cake: Once the cake layers are cool, carefully slice each layer in half horizontally to create a total of four layers. Soak each layer with the prepared coconut simple syrup, then add a layer of coconut custard filling. Repeat with the remaining layers, then frost the top and sides with the buttercream and generously coat with the toasted coconut flakes.