Preheat your oven to 350°F (175°C). Grease and flour three 6-inch cake pans, then line them with parchment paper. In a medium bowl, whisk together the flour, baking powder, spices, and salt. Set this dry mixture aside. In the bowl of a stand mixer, combine the vegetable oil with both sugars and beat for about 2 minutes until well blended. Add the eggs one at a time, making sure each is fully incorporated before adding the next. Stir in the vanilla extract. Add the flour mixture in three parts, mixing gently to combine. Fold in the cinnamon-sugar coated apples. Divide the batter evenly among the prepared pans, smoothing the tops. Bake for 45–50 minutes or until a toothpick inserted into the center comes out mostly clean. Let the cakes cool in the pans on a wire rack for 10 minutes, then carefully remove them and cool completely on the rack.

Caramel:

In a medium saucepan, combine the granulated sugar and water, stirring just to mix. Do not stir again after this point. Cook over high heat, occasionally rinsing down the sides of the pot with a wet pastry brush to prevent sugar crystals from forming. When the mixture reaches a deep amber color, immediately remove it from heat. Slowly pour in the heavy cream while whisking vigorously—the mixture will bubble and boil intensely. Add the butter, then return the pan to medium heat and bring back to a boil, whisking constantly. Let it cook for 2 minutes until smooth and thickened. Remove from heat and allow the caramel to cool to room temperature. Transfer it to a container and refrigerate until thickened.

Caramel Swiss Meringue Buttercream:

Combine the egg whites and granulated sugar in the bowl of a stand mixer and whisk to combine. Place the bowl over a pot of simmering water (double boiler) and whisk constantly until the mixture reaches 160°F (71°C) on a candy thermometer and feels smooth and no longer grainy, about 3 minutes. Transfer the bowl back to the mixer and whip on medium-high speed until stiff peaks form and the bowl feels cool to the touch, about 5 to 10 minutes. Switch to the paddle attachment and add the cubed butter gradually, mixing until the frosting is smooth and creamy. Finally, mix in the cooled caramel until fully incorporated and silky.

Assembly:

If necessary, level the cake layers with a serrated knife. Place one cake layer on a serving plate or cake stand and spread about 2/3 cup of the caramel buttercream evenly on top. Repeat with the remaining layers. Apply a thin crumb coat of frosting around the outside of the cake and chill for 20 minutes to set. Using a small spoon, drop dollops of caramel around the top edges of the chilled cake, allowing some caramel to drip down the sides. Pour the remaining caramel over the top of the cake and spread evenly with an offset spatula for a smooth finish.