Put 1/2 stick butter into a large nonstick skillet. Using the tip of a paring knife, scrape seeds from vanilla bean into skillet, and toss in pod. Cook over medium heat until butter is lightly browned, about 2 minutes. Stir in apples, cider, and 1 cup granulated sugar. Raise heat to medium-high, and cook, stirring occasionally, for 8 minutes. Reduce heat to low, and cook until apples are golden and translucent and liquid has evaporated, about 10 minutes. Discard vanilla pod. Let cool.
Preheat oven to 350 degrees. Butter two 8-by-2-inch round cake pans. Dust with flour, and tap out excess. Whisk together flour, baking powder, baking soda, salt, and spices. Reserve 1/4 cup flour mixture.
Beat remaining 2 sticks butter, remaining 1 1/2 cups granulated sugar, and the brown sugar with a mixer on medium-high speed until pale and fluffy, about 5 minutes. Reduce speed to medium, and add eggs, 1 at a time, beating well after each addition. Reduce speed to low. Beat in flour mixture in 2 batches, alternating with the sour cream and scraping bowl as needed. Add remaining 1/4 cup flour mixture to apple mixture, and toss. Fold in apple mixture and pecans.
Divide batter between prepared pans. Bake until tops are dark golden brown and a toothpick inserted into centers comes out clean, 55 to 65 minutes. Let cool for 30 minutes on a wire rack. Run a knife around edges of cakes to loosen. Unmold cakes, and let cool, right side up, on rack. (Cakes can be wrapped in plastic and refrigerated for up to 3 days.)
Assemble cake: Trim each cake so it's 1 3/4 inches high. Place 1 cake, cut side up, on a cake stand. Spread 1 1/2 cups buttercream over top. Place remaining cake, cut side down, on top. Spread 1 cup buttercream over entire cake. Refrigerate until firm, about 45 minutes. Spread remaining frosting over cake. (Cake can be refrigerated, uncovered, overnight.)
Before serving, decorate with marzipan pumpkins, acorns, and leaves.