In a medium saucepan over medium heat, combine 1 cup of whole milk and 1 cup of canned unsweetened coconut milk, heating until just steaming but not boiling. While that warms, whisk together 4 large egg yolks, ¾ cup granulated sugar, and ¼ cup cornstarch until smooth. Gradually whisk the warm milk mixture into the egg mixture, return to heat, and stir continuously until thickened (about 5-7 minutes). Remove from heat and mix in 2 tablespoons of coconut rum and 1 teaspoon of vanilla extract. Pour into a bowl, cover, and chill in the refrigerator for at least 2 hours.

Preheat your oven to 350°F (175°C) and prepare three 9-inch round cake pans by greasing them and lining the bottoms with parchment paper. In a large bowl, whisk together 2 cups of gluten-free all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. In another bowl, cream 1 cup of unsalted butter with 1½ cups granulated sugar until light and fluffy. Beat in 4 egg whites, and then alternate mixing in the dry ingredients with 1 cup of whole milk and ½ cup of yogurt, beginning and ending with the dry ingredients.

Divide the cake batter evenly among the prepared pans, smoothing the tops with a spatula. Bake in your preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out clean, and the edges are lightly golden. Once baked, remove the pans from the oven and let them cool for about 10 minutes in the pans. Then, turn them out onto a wire rack to cool completely.

While the cakes cool, prepare the toasted coconut. Preheat your oven to 350°F (175°C) if you’ve turned it off. Spread 2 cups of sweetened flaked coconut evenly on a baking sheet. Bake for 5-7 minutes, stirring occasionally, until the coconut is golden brown and fragrant. Set aside to cool.

In a small saucepan, bring ½ cup of water and ½ cup of granulated sugar to a boil. Once boiling, stir in 1 cup of sweetened flaked coconut, then remove from heat. Let it steep for about 15 minutes, then strain the syrup into a bowl and let it cool to room temperature.

For the frosting, beat 1 cup of unsalted butter in a large bowl until creamy and light. Gradually add 3-4 cups of powdered sugar, mixing until well combined. Slowly blend in 1 cup of chilled coconut custard until smooth and fluffy.

Once the cake layers are completely cool, use a serrated knife to slice each layer in half horizontally, creating a total of six layers. Place one layer on a serving plate, brush generously with coconut simple syrup, and spread an even layer of coconut custard on top. Repeat with the next layers, frosting each with the creamy frosting.

Once assembled, coat the entire cake generously with the remaining frosting. Use a spatula to create a smooth finish or decorative swirls. Finally, generously sprinkle the toasted coconut on top.