This recipe is for a classic, one-tiered layer cake. Clear vanilla extract keeps the buttercream pure white; it's available from Wilton
and King Arthur
To make cake:
Adjust 2 oven racks to divide oven into thirds. Preheat oven to 350°. Grease 3 (9" x 1 1/2") round cake pans. Line bottoms with parchment paper; grease paper and dust pans with flour, tapping out excess.
Sift flour, baking powder, and salt into a bowl. Combine milk, 1/2 cup water, and vanilla in another small bowl.
In stand mixer with paddle attachment, beat butter on low speed until creamy. Add 1 3/4 cups sugar and beat on medium speed until fluffy, about 4 minutes, scraping bowl occasionally. With mixer on low speed, add flour mixture in thirds, alternating with milk mixture, beginning and ending with flour mixture. Beat just until smooth, scraping bowl with rubber spatula occasionally. Transfer batter to large bowl.
In a clean bowl of stand mixer with whisk attachment, whisk egg whites on medium speed until foamy. Gradually whisk in remaining 1/4 cup sugar. With mixer on medium-high speed, whisk until soft peaks form. In 2 batches, gently fold egg whites into batter with a large rubber spatula, just until no white streaks remain. Divide batter between prepared pans and spread evenly.
Stagger pans on oven racks, placing 2 layers on bottom rack, and 1 on top rack, so that pans are not directly above one another. Bake 20 to 25 minutes, or until a toothpick inserted in center of cake comes out clean. Cool pans on a rack 10 minutes. Run a knife around sides of pans and turn out cakes onto racks; remove parchment. Flip cakes and let cool completely.
To make buttercream filling:
Combine sugar and 1/3 cup water in a small saucepan and bring to a boil over medium-high heat, stirring to dissolve sugar. Then boil without stirring until syrup reaches 240° on a digital thermometer, about 6 minutes.
Meanwhile, in stand mixer with whisk attachment, beat egg yolks on medium speed until slightly thickened, about 2 minutes. With mixer running, gradually pour in hot syrup in a thin stream; avoid pouring syrup on whisk. Increase speed to medium-high and beat until yolk mixture is thick and cool, about 8 minutes. With mixer on medium speed, add the butter 1 tablespoon at a time, beating until blended after each addition. (If at any time buttercream appears curdled, increase speed to high and beat until smooth; then reduce speed to medium and continue beating in butter.) Divide buttercream evenly between two bowls; stir coconut into one bowl, and blood orange zest and juice into other.
To make buttercream icing:
Combine sugar and 1/2 cup water in a small saucepan and bring to a boil over medium-high heat, stirring to dissolve sugar. Boil without stirring until syrup reaches 240° on a digital thermometer, about 6 minutes.
Meanwhile, in a stand mixer with whisk attachment, beat egg whites on medium-high speed until soft peaks form. With mixer on medium speed, gradually pour in hot syrup in a thin stream; avoid pouring syrup on whisk. Increase speed to medium-high and beat until stiff peaks form and mixture is cool, about 8 minutes. Reduce speed to medium and add butter 1 tablespoon at a time, beating after each addition. (If at any time buttercream appears curdled, beat on high until smooth, then reduce speed to medium and continue beating in butter.) Once all butter is added, beat on high speed until buttercream is smooth and fluffy, about 1 minute. Beat in lime zest, lime juice, and vanilla.
To assemble cake:
Place 1 cake layer on cake stand or serving plate. Spread with coconut buttercream filling. Top with another cake layer; spread with orange buttercream filling. Top with last cake layer, top-side up. Spread sides and top of cake with about 1 cup buttercream icing to crumb coat cake. Refrigerate cake 15 minutes. Frost sides and top of cake with remaining buttercream icing. If not serving immediately, refrigerate. If cake is refrigerated, let stand at room temperature for 1 hour before serving.