To prepare the apple filling:
- In a medium saucepan over medium heat, cook the butter, stirring continuously until it melts and barely begins to turn golden brown. Immediately add the apples and stir to combine. Add the remaining ingredients, bring to a simmer, and cook for 5 minutes or until the juices have thickened slightly and the apples have barely softened. Remove the filling to a heat-safe bowl and cool completely prior to using.
To prepare the cake:
- Preheat the oven to 350 degrees. Grease the sides and line the bottoms of 3-8” round cake pans with parchment paper and set aside.
- In a small bowl, combine the cake flour, baking powder, and salt and set aside.
- In a large bowl or the bowl of a stand mixer, cream together the butter, sugar, and brown sugar on medium speed until light and fluffy, about 2 minutes. Scrape the sides of the bowl with a spatula and add each egg one at a time on low speed, mixing just until combined. Scrape the sides of the bowl. Add about 1/3 of the dry ingredients followed by the vanilla and about half of the buttermilk. Mix until combined and then repeat this process once more, finishing by adding the last third of the dry ingredients. Scrape the sides of the bowl and fold in any unincorporated bits of batter.
- Distribute the batter evenly among the three pans and bake in the preheated oven 20-25 minutes or until a toothpick inserted comes out clean. Allow to cool completely prior to assembling cake.
To prepare the maple buttercream:
- Cream the butter in the bowl of a stand mixer or a large mixing bowl on medium speed until light and fluffy, about 2 minutes. Add the brown sugar and cream for an additional minute. Scrape the sides of the bowl and add the confectioner’s sugar. Mix on low speed until incorporated and then drizzle in the maple syrup. Beat on medium speed until well combined. If needed, add a bit of water to thin out frosting or add a bit more powdered sugar to thicken up. You can also briefly place the frosting in the fridge to firm up as needed.
To assemble the cake:
- Use a serrated knife to level the cakes. Smooth a small amount of frosting on an 8” cake board or plate and center a single cake layer on top. Smooth a thin layer of frosting on top of the cake layer. Using a piping bag (see notes) fitted with a large round tip, pipe a dam around the outer rim of the top of the cake. The dam should be at least ¼-1/2” tall to prevent the apple filling from squirting out the sides. Spread approximately half of the apple filling inside the dam and sprinkle with about 1/3 of the nuts. Pipe a bit of frosting on top of the apple filling to help the next layer of cake stick. Stack the second cake layer on top and repeat the entire process once more. Place the final cake layer on top. Spread a thin coat of frosting (crumb coat) all over the cake and allow it to set up in the fridge prior to applying the final coat of frosting. You can skip this step if desired. Decorate the top of the cake with a few nuts and extra frosting, if desired. Cake will keep in the refrigerator covered in plastic for up to three days.