Preheat oven to 350 degrees F. Line standard muffin tins with paper liners.: Warmth in the oven creates the environment for a controlled rise, so preheat thoroughly until the thermometer reads 350 degrees F , and the racks are in the middle. You should hear a faint hum from the oven as it maintains temperature, and the air takes on a little dry heat. If liners are used, they help with even release and neat presentation. A common mistake is not preheating long enough, which can cause uneven rise and dense centers. Give your oven time to stabilize, and ensure the tins are at room temperature before filling to avoid thermal shock.
In a medium bowl, whisk together the flour, baking powder and salt. Pulse the shredded coconut in a food processor until finely ground and whisk into the flour mixture; set aside.: The dry mix should look even and pale when well combined, with the ground sweetened shredded coconut dispersed. Whisking aerates the flour slightly and distributes the baking powder so the lift will be consistent. The pulsed coconut will be fragrant and integrate without big flecks, adding a concentrated coconut note. If you skip pulsing, you might get uneven pockets of coconut, and that can alter texture. Be tidy here to avoid overworking the gluten once wet ingredients are added.
With an electric mixer on medium-high speed, cream the butter and sugar together until pale and fluffy, about 3 minutes. Add the egg, egg white and vanilla extract and beat until completely combined, scraping down the sides of the bowl as needed. Reduce the mixer speed to low. Add the flour mixture in three batches, alternating with two additions of coconut milk, and beating until combined after each.: You should watch the unsalted butter and granulated sugar turn from gritty to glossy and pale; this indicates air trapped in the fat for lift. When you add the egg and egg white , the batter will glossy and smell richer. Alternating the dry mix with the unsweetened coconut milk keeps the batter emulsified and prevents curdling, resulting in a silky texture. Listen for a steady motor sound from the mixer, and scrape the bowl to avoid streaks of dry flour. A common misstep is adding all the liquid at once, which can deflate the mixture and yield dense cupcakes, so alternate carefully.
Divide the batter evenly between the lined cups. Bake until a toothpick inserted in the center comes out clean, 18 to 20 minutes. Remove from the oven, let cool in the pan for 10 minutes, then remove to a wire rack to cool completely.: As the cupcakes bake, their tops will become faintly golden and spring back when touched lightly. You may hear gentle crackles as steam escapes. The toothpick test ensures the interior is set while remaining moist. Cooling in the pan for a short period prevents the fragile cakes from collapsing, then transferring to a wire rack stops residual heat from overcooking them. A common error is leaving them in the hot pan too long, which can oversteam the bottoms; follow the ten minute guideline to keep the texture balanced.
Make the Frosting: Using the whisk attachment of a stand mixer, whip the butter on medium-high speed for 5 minutes, stopping to scrape the bowl once or twice. Reduce the speed to low and gradually add the powdered sugar. Once all of the powdered sugar is incorporated, increase the speed to medium-high and add the vanilla and coconut extracts, mixing until incorporated. Whip at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl as needed.: Using the whisk attachment of a stand mixer, whip the butter on medium-high speed for 5 minutes, stopping to scrape the bowl once or twice. Reduce the speed to low and gradually add the powdered sugar. Once all of the powdered sugar is incorporated, increase the speed to medium-high and add the vanilla and coconut extracts, mixing until incorporated. Whip at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl as needed. : The butter will transform into a cloud like texture as air is whisked in, and it should smell sweet and buttery without being greasy. Adding the powdered sugar gradually prevents a gritty or dusty frosting; you want a smooth ribbon when the beater lifts. The vanilla extract and coconut extract bloom the aroma, and whipping at higher speed yields a glossy, spreadable buttercream. If the frosting seems too soft, chill briefly; if too stiff, a splash of coconut milk softens it. Overwhipping can incorporate too much air and make the frosting unstable, so stop when it looks light and stable.
Garnish the Cupcakes: Using an offset spatula, spread a mound of frosting on top of each cupcake and create a well in the center. Place the toasted coconut in a shallow bowl. Roll the sides of the cupcake in the toasted coconut, then top with a few Robin's Eggs malted milk balls. The cupcakes should be stored in an airtight container at room temperature for up to 3 days.: Using an offset spatula, spread a mound of frosting on top of each cupcake and create a well in the center. Place the toasted coconut in a shallow bowl. Roll the sides of the cupcake in the toasted coconut, then top with a few Robin's Eggs malted milk balls. The cupcakes should be stored in an airtight container at room temperature for up to 3 days. : When you spread the frosting, the surface should be glossy and hold the impression of the spatula. Rolling the sides in toasted coconut adds a crunchy rim that contrasts with the soft frosting and cake. The Whoppers Robin’s Eggs are optional, and they add color and a different crunch if you choose them. Store at room temperature away from heat so the buttercream stays stable. A typical mistake is refrigerating them uncovered, which can dry the cake and cause the frosting to stiffen, so use an airtight container to maintain texture.