Optional: large coconut flakes and toasted whole almonds
Preheat oven to 325°. Grease and flour two 9-in. round baking pans. In a large bowl, combine cake mix, buttermilk, eggs and oil; beat on low speed 30 seconds. Beat on medium 2 minutes. Divide batter between prepared pans. Bake until a toothpick inserted in the center comes out clean, 28-32 minutes. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For filling, in a large saucepan, combine marshmallows, sugar, milk and cornstarch. Cook and stir over low heat until marshmallows are melted; increase heat to medium and bring to a boil, stirring constantly. Stir in coconut and almonds. Remove from heat; cool. Refrigerate filling until cold but still spreadable, about 45 minutes.
Using a long serrated knife, cut each cake horizontally in half. Place 1 cake layer on a serving plate. Spread with 1/3 of coconut filling. Repeat twice. Top with remaining cake layer. Spread fudge frosting over top and sides of cake. Refrigerate until set, at least 1 hour. Garnish with coconut flakes and almonds if desired. Refrigerate leftovers.