Step 1: Preheat your oven to 350°F (175°C). Prepare three 8-inch round cake pans by greasing them thoroughly and lining the bottoms with parchment paper for easy release.
Step 2: In a medium bowl, whisk together the dry ingredients: cake flour, kosher salt, baking powder, and baking soda. Set this mixture aside.
Step 3: In a separate measuring jug, combine the apple cider, whole milk, and grapeseed oil. Stir well and set aside.
Step 4: Using the bowl of a stand mixer fitted with the paddle attachment, beat the softened unsalted butter with the granulated sugar and vanilla extract until the mixture is light, fluffy, and smooth.
Step 5: Add the egg yolks one at a time, mixing well after each addition. Then, add the whole eggs, one at a time, ensuring each is fully incorporated before adding the next, scraping down the sides of the bowl as needed.
Step 6: Begin combining the wet and dry ingredients with the creamed butter mixture. Alternate additions, starting with one-quarter of the dry mixture, mixing until just combined, then adding one-third of the wet mixture. Repeat this process until all ingredients are incorporated, being careful not to overmix.
Step 7: Divide the cake batter evenly among the three prepared cake pans. Smooth the tops with a spatula. Bake for 35-40 minutes, or until a toothpick inserted into the center of a cake comes out clean. Let the cakes cool in their pans for 5 minutes before carefully transferring them to a wire rack to cool completely.
Step 8: While the cakes are cooling, prepare the apple filling. In a saucepan over medium heat, melt the unsalted butter. Add the chopped Granny Smith and Gala apples, lemon juice, brown sugar, cinnamon, and apple cider. Simmer for 5-7 minutes, stirring occasionally, until the apples are softened but still hold their shape. Stir in the arrowroot powder and continue to cook for another minute until thickened. Remove from heat and let the filling cool completely.
Step 9: Next, make the apple cider buttercream. In a clean stand mixer bowl with the paddle attachment, beat the softened unsalted butter until it's smooth and creamy. Gradually add the powdered sugar, apple cider concentrate, vanilla extract, and kosher salt. Beat on medium-high speed until the buttercream is light, fluffy, and smooth. Adjust consistency with a little more concentrate if needed.
Step 10: Once the cake layers are completely cool, level them using a serrated knife if necessary. Place one cake layer on your serving plate or cake stand. If using the Calvados soaking liquid, drizzle a small amount evenly over the cake layer. Spread a generous layer of apple cider buttercream over the cake, then pipe a border of buttercream around the edge. Fill the center with half of the cooled apple filling.
Step 11: Carefully place the second cake layer on top and repeat the soaking, buttercream, and filling process. Place the final cake layer on top. Apply a thin layer of buttercream all over the cake (the crumb coat) and refrigerate for 15-30 minutes to set. Once set, apply the remaining buttercream, creating a smooth, even finish. Decorate as desired with piped details or a sprinkle of cinnamon.
Step 12: For the best flavor and texture, allow the assembled cake to sit at room temperature for about 15 minutes before slicing and serving. Enjoy your homemade taste of fall!