Reduce the apple cider: Stirring occasionally, simmer the apple cider in a large saucepan over medium-low heat until you’re left with about 1½ cups. Start checking at 30 minutes, 35 minutes, 40 minutes, etc until you have 1½ cups. Set aside to cool for 10 minutes.
Preheat oven to 350°F (177°C). Grease three 8-inch cake pans, line the bottoms with parchment paper, then grease the parchment paper.
Make the apple cider cake: In a medium bowl, whisk together the eggs, egg yolks, vanilla, sour cream, and 1⅓ cups (330 g) of the reduced apple cider. Set aside.
In the In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugars, baking powder, cinnamon, and salt. Mix briefly to combine. Then, with the mixer on low, gradually add the butter one cube at a time, until fully incorporated and the mixture resembles wet sand. Add half of the wet ingredients and mix until just combined. Add the second half of the wet ingredients and mix on low speed until just combined and there are no pockets of flour.
Divide the batter between your three cake tins and bake for 35 to 40 minutes, or until the tops of the cakes spring back when gently touched and a toothpick inserted in the center comes out clean. If the cakes need a little longer as determined by wet crumbs on the toothpick, bake for longer. However, careful not to overbake as the cakes may dry out. Remove cakes from the oven and cool for 15 minutes in the pants before allowing to cool completely on a wire rack. The cakes must be completely cool before frosting and assembling.
Make the spiced salted caramel: Add the heavy cream, cinnamon, ginger, cardamom, cloves, and nutmeg to a glass measuring cup and heat in the microwave for 30 seconds, then set aside. In a medium saucepan, heat the sugar on medium-low heat for about 5 minutes. Use a spatula to occasionally stir the sugar, ensuring the layer on the bottom of the pan isn't burning. Once the sugar is fully melted and turns a golden/amber hue, remove from heat immediately and stir in butter. The mixture will violently bubble. Use your spatula to stir the butter and sugar until they're fully combined. Next, pour in the spiced heavy cream mixture. Use your spatula to stir until everything is combined. Place back on the stove for about 30 seconds, stirring the whole time. Remove from the heat, stir in the salt, and allow to cool at room temperature while you make your frosting.
Make the spiced salted caramel Swiss meringue buttercream: In the bowl of an electric stand mixer, whisk the egg whites and granulated sugar together briefly by hand, just until they are combined. Fill a sauce pan with a few inches of water and bring to a simmer. Place the mixer bowl with the egg white mixture on top to create a double-boiler and heat the mixture until it reaches 160°F (70°C) on a candy thermometer and the sugar has dissolved, stirring often.
Carefully return the bowl to the stand mixer. Fitted with the whisk attachment, beat the mixture on high speed for about 8 minutes. When done, the meringue should hold shiny, medium-stiff peaks and be cooled to room temperature. With the mixer on low, begin adding in the butter a couple tablespoons at a time. If it looks curdled, just keep mixing! Once the butter has been fully incorporated, add in the vanilla bean paste and the spiced salted caramel and beat on high until combined, about 2-3 minutes. Then switch to the paddle attachment and mix on low for one minute to remove any air.
Assemble and frost: Using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Place a cake layer on your cake stand or serving plate and add about 1 cup of spiced salted caramel buttercream onto the first layer of cake; smooth using an offset spatula.
Repeat with the 2nd layer. Top with the 3rd layer of cake upside down, spreading buttercream all over the top and sides to crumb coat the cake. Smooth using a cake scraper or an offset spatula, scraping off the scraper between passes, and placing the scraped off buttercream into a bowl off to the side. Chill in the refrigerator for 15-20 minutes. Once chilled, frost the cake with the remaining buttercream - being sure to reserve some for decoration on top, if desired. (We piped the blobs on top using Ateco #866 star tips. For the ombré effect, we whipped 500g of the spiced salted caramel buttercream with 200g of melted dark chocolate for the dark-colored buttercream, and then mixed some of that dark brown-colored buttercream with the spiced salted caramel buttercream for the medium brown-colored buttercream.)