John's Three Layer Apple Cake

Found at marthastewart.com

• Ready In: 3 hrs 5 mins
• Serves: 12

Ingredients

• FOR THE CAKE

• 1⁄2 cup unsalted butter, melted, plus more for pans
• 2  cups all-purpose flour, plus more for pans (spooned and leveled)
• 2  teaspoons baking soda
• 1⁄2 teaspoon baking powder
• 2  teaspoons ground cinnamon
• 1⁄2 teaspoon ground ginger
• 3⁄4 teaspoon salt
• 2  cups  packed light-brown sugar
• 2  large eggs
• 4  granny smith apples, peeled, two coarsely grated and two diced

• FOR THE BUTTERCREAM

• 4  large  egg whites
• 1  cup  packed light-brown sugar
• 1⁄4 teaspoon salt
• 1 1⁄2 cups unsalted butter, room temperature, cut into pieces

Directions

1. Preheat oven to 350 degrees. Butter three 8-inch-round cake pans; line bottoms with parchment paper. Butter, then flour paper and sides (tapping out excess); set aside.
2. In a medium bowl, whisk together flour, baking soda, baking powder, cinnamon, ginger, and salt; set aside. In a large bowl, whisk together butter, sugar, and eggs until well combined; fold in grated and diced apples. Add flour mixture; mix just until combined. Divide batter among prepared pans; smooth tops.
3. Bake until a toothpick inserted in center of cake comes out clean, 35 to 40 minutes. Cool cakes in pans 20 minutes, then invert onto wire racks; peel off parchment. Invert cakes again, and let cool completely on racks.
4. Make buttercream: In a heatproof bowl set over (not in) a pan of simmering water, whisk together egg whites, sugar, and salt until sugar has dissolved; remove from heat. Using an electric mixer on medium, beat mixture until fluffy and cooled, about 15 minutes. Increase to high; beat until stiff peaks form. Reduce to medium-low; gradually add butter, beating until fully incorporated, about 3 to 5 minutes.
5. Assemble cake: Place one layer on a serving plate; spread top with buttercream. Top with another layer; spread top with buttercream. Top with third layer; spread remaining buttercream over top and sides of cake.
6. Refrigerate at least 1 hour (or up to 4 days). To serve, bring cake to room temperature; slice with a serrated knife.
7. Helpful Hint:.
8. You can also bake the cake in a 13-by-9-inch pan; the baking time will be about the same. Also, if the buttercream separates, continue beating -- it will come together.