Smith Island Cake

Place chocolate in large bowl. Heat cream, sugar, and salt in saucepan over medium-low heat, stirring occasionally, until sugar dissolves and mixture begins to simmer. Pour hot cream mixture over chocolate and whisk until smooth. Whisk in vanilla and butter until glossy. Cover and refrigerate until icing is firm but still spreadable, about 1 hour.

Adjust oven rack to middle position and heat oven to 350 degrees. Generously grease two 9-inch round cake pans, line with parchment paper, grease parchment, and flour pans. Whisk flour, baking powder, baking soda, salt, and 1½ cups sugar together in bowl. Whisk buttermilk, melted butter, egg yolks, oil, and vanilla together in second bowl.

Using stand mixer fitted with whisk attachment, whip egg whites and cream of tartar on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip whites to soft billowy mounds, about 1 minute. Gradually add remaining ¼ cup sugar and whip until glossy, stiff peaks form, 2 to 3 minutes; transfer to third bowl.

Add flour mixture to now-empty mixer bowl and mix on low speed, gradually adding buttermilk mixture and mixing until almost incorporated (a few streaks of dry flour will remain), about 15 seconds. Scrape down bowl, then mix on medium-low speed until smooth and fully incorporated, 10 to 15 seconds.

Using rubber spatula, stir one-third of whites into batter. Gently fold remaining whites into batter until no white streaks remain. Spread about ⅔ cup batter in even layer in each prepared pan. Bake until edges are golden brown and cake springs back when touched, 10 to 14 minutes. Let cakes cool in pan on wire rack for 5 minutes. Run thin knife around edge of pans, remove cakes from pan, discarding parchment, and let cool completely on rack. Let pans cool completely, reline pans with parchment, grease parchment, and flour pans. Repeat process 3 times for a total of 8 layers.

Line edges of cake platter with 4 strips of parchment to keep platter clean. Place 1 cake layer on platter. Spread ¼ cup frosting evenly over top, right to edge of cake. (If frosting is too stiff, let stand at room temperature 5 minutes, then stir to soften.) Repeat with 6 more cake layers, spreading ¼ cup frosting evenly over each layer. Top with remaining cake layer and spread remaining frosting evenly over top and sides of cake. To smooth frosting, run edge of offset spatula around cake sides and over top. Carefully remove parchment strips before serving.