Preheat oven to 350°F / 175°C. Line three 8" pans with parchment rounds and grease with non-stick baking spray.

Whisk flour, brown sugar, granulated sugar, baking powder, salt, cinnamon, ginger, and nutmeg in a large bowl or the bowl of a stand mixer.

Mix butter slowly into the dry ingredients on low speed until no large chunks remain.

In a separate bowl, whisk together eggs, sour cream, vegetable oil, and vanilla extract.

Slowly mix wet ingredients into dry ingredients on low speed until just incorporated. Scrape down the sides of the bowl and beat for a few seconds.

Divide the batter evenly between the prepared pans and bake for 32-35 minutes or until a toothpick comes out with a few moist crumbs.

Allow the pans to cool for 10 minutes, then run a small spatula around the perimeter of the pan to separate the cake from the pan. Flip the cake layers onto a wire rack to cool completely. Level the tops with a serrated knife.

For the apple filling, peel and core apples, then cut into quarter-inch pieces. In a large skillet, add apples, butter, brown sugar, cinnamon, and salt; heat over medium-high heat until bubbling, then reduce to medium.

Whisk together water and cornstarch to make a slurry. Pour into the apple mixture and simmer for 5-10 minutes until the apples are soft. Transfer filling to a container and chill in the fridge.

For the caramel sauce, turn stove to medium heat and add sugar gradually, stirring until melted and amber colored.

Slowly mix in butter, then cream and salt. Pour into a container and cool in the fridge for 20 minutes.

For the buttercream frosting, beat butter until smooth, mix in cooled caramel, vanilla, and salt, then gradually incorporate powdered sugar. Add heavy cream halfway through mixing.

Assemble the cake by stacking and frosting the spiced cake layers. Spread caramel buttercream on the first layer, pipe a ring of frosting dam, add apple filling and drizzle with caramel. Repeat with remaining layers.

Chill the assembled cake in the freezer for 5 minutes to lock in the filling.

Spread a thick coat of caramel buttercream around the cake for decoration.