Prepare the Pans: Preheat your oven to 175°C (350°F). Grease three 9-inch round cake pans generously with butter, then dust lightly with flour. Line the bottom of each pan with a circle of parchment paper for easy release.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Cream Butter and Sugar: In a large bowl, beat the softened butter and sugar together with an electric mixer on medium-high speed for 4-5 minutes until the mixture is pale, light, and fluffy. This step is crucial - proper creaming incorporates air for a tender crumb.
Add Egg Whites: Add the egg whites one at a time, beating well after each addition. Then mix in the vanilla extract and coconut extract (if using). The batter may look slightly curdled after adding the whites - this is normal and will smooth out when you add the flour.
Alternate Dry and Wet: With the mixer on low speed, add the flour mixture in three additions, alternating with the milk in two additions, beginning and ending with the flour. Mix just until combined after each addition - do not overmix. Gently fold in the shredded coconut and orange zest (if using) with a spatula.
Bake: Divide the batter evenly among the three prepared pans. Smooth the tops with an offset spatula. Bake for 25-30 minutes, or until a toothpick inserted in the centre comes out clean and the cakes spring back when lightly pressed. Cool in pans for 10 minutes, then invert onto wire racks and cool completely.
Make the Seven-Minute Frosting: Combine the sugar, egg whites, water, corn syrup, and cream of tartar in the top of a double boiler (or a heatproof bowl set over a pot of simmering water). Beat with an electric hand mixer on low speed for 1 minute to combine. Then increase to high speed and beat continuously for about 7 minutes, until the frosting reaches 70°C (160°F) on a candy thermometer and forms stiff, glossy peaks. Remove from heat, add vanilla extract, and beat for 2 more minutes until the frosting is thick, fluffy, and holds its shape.
Assemble the Cake: Place the first cake layer on your serving plate or cake board. Spread about ¾ cup of frosting evenly over the top. Sprinkle a thin layer of shredded coconut. Repeat with the second layer. Place the third layer on top and frost the entire top and sides of the cake with the remaining frosting, using an offset spatula to create a smooth or swoopy finish.
Add the Snow Coating: While the frosting is still soft and sticky, generously press and sprinkle shredded coconut all over the top and sides of the cake. Use your hands to gently pat the coconut onto the sides so it sticks. The coconut should cover the entire cake, creating that signature snowy white appearance.
Serve: Let the cake rest for 15-20 minutes before slicing. For the cleanest cuts, use a large sharp knife dipped in warm water and wiped dry between each slice.