Preheat oven to 350F. Line 2, 8-inch cake rounds with parchment paper. In a large mixing bowl, beat together softened butter, sugar, maple and vanilla extracts until fluffy. Add the eggs and beat again until creamy and smooth. Next, pour in the milk (I recommend using warm milk).
In a separate bowl, combine all the dry ingredients: flour, baking powder and salt. Sift the dry ingredients into the cake batter and whisk by hand to avoid over-mixing the batter. Add the pecans and fold them in last.
Divide the batter evenly between the two pans. Bake in preheated oven for about 40 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool completely on wire racks, then using a serrated knife, split each layer in half.
For Apple Pie Filling:
Meanwhile, prepare the apple pie filling. Peel and dice 4 large apples. Preheat a large pan and add the apples, water and white granulated sugar. Cover the pan and bring the apples up to a simmer for 5 minutes.
Add the remaining ingredients, except for butter: brown sugar, vanilla, nutmeg and cinnamon. Continue cooking the apples over medium heat until apples are tender and sugar begins to caramelize, about 15 minutes. Add the butter last and allow it to melt in the pan. Remove the filling from heat and allow it to cool completely before using to fill the cake.
For Salted Caramel Buttercream:
Next, prepare the frosting: Place softened, unsalted butter into mixing bowl. Whisk on high speed for about 5 minutes, until butter is light and fluffy. Scrape down sides of mixing bowl often to get all the butter mixed well. Add the dulce de leche (cooked sweetened condensed milk). If you cannot find dulce de leche, use a similar caramel sauce. If necessary, increase the amount of confectioner's sugar to thicken the frosting.
Add the vanilla and salt. Start by adding 1/4 teaspoon salt, then add more if needed. Begin adding the confectioner's sugar 1 cup at a time, scraping down sides of mixing bowl often to get everything incorporated. Make sure to sift confectioner's sugar before adding to avoid any clumps. After adding all sugar, whisk on high speed for 2 to 3 minutes to get frosting very fluffy. Transfer half the frosting into a pastry bag tipped with a large star tip; I used Ateco tip #847.
Assembling the Cake:
Assemble the cake. Pipe a barrier ring of frosting around the edge of each layer before adding the apple pie filling into the center. Repeat for the first 3 layers. Top with the final layer, then use the remaining buttercream to frost the top and sides of cake. Allow the cake to chill in the refrigerator for about 30 minutes before garnishing the top.
Chocolate Ganache:
If desired, garnish the cake with a simple chocolate ganache. Heat the heavy cream in the microwave until steaming hot, then add the chocolate. Stir to melt completely until a smooth ganache forms. Allow the ganache to cool at room temperature to thicken. Pour over the top of the cake and garnish the cake as desired.