In a medium saucepan, combine heavy cream, sugar, and cornstarch over medium heat. Stir constantly until thick and bubbling, about 5-7 minutes. Remove from heat and mix in vanilla extract and shredded coconut. Allow to cool, stirring occasionally.

In a large mixing bowl, cream together unsalted butter and sugar using an electric mixer until light and fluffy, around 3-4 minutes. Gradually add room temperature eggs, one at a time, mixing well after each addition.

In another bowl, sift together flour, baking powder, and salt; alternate adding dry mix and warm water to butter mixture until fully combined.

Preheat oven to 350°F (175°C) and grease four 8-inch cake pans. Divide batter among pans and smooth tops. Bake for 25-30 minutes, or until a toothpick comes out clean.

Once baked, let layers cool in pans for about 10 minutes. Transfer to a wire rack to cool completely. Meanwhile, prepare frosting by beating cream cheese, sugar, coconut extract, and heavy cream until smooth.

To assemble, place one layer on a serving plate and spread custard filling on top. Repeat with remaining layers. Frost the entire cake with cream cheese frosting and sprinkle toasted coconut on top.