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

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

Mix in 1/2 cup of unsalted butter slowly into the dry ingredients on low speed until no large chunks of butter remain.

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

Slowly mix the wet ingredients into the dry ingredients on low speed until just incorporated.

Scrape down the sides of the bowl, then beat on low speed for a few seconds to ensure everything is mixed.

Divide the batter evenly between the prepared pans.

Bake for 32-35 minutes or until a toothpick comes out with a few moist crumbs. Allow to cool for 10 minutes, then run a spatula around the pan to separate the cake from the pan.

Flip the cake layers onto a wire rack to cool. Level the tops of the layers with a serrated knife once cool.

For the apple filling, peel and cube the apples. In a skillet, combine apples, butter, brown sugar, cinnamon, and salt. Heat over medium-high until bubbling, then reduce heat.

Whisk together water and cornstarch to make a slurry, then pour into the apple mixture. Simmer for 5-10 minutes until apples are soft. Chill the filling in the fridge.

For the caramel sauce, melt sugar gradually in a pot over medium heat until it turns amber. Mix in butter, then cream and salt. Pour into a container and cool in the fridge for 20 minutes.

For the buttercream, beat butter until smooth. Mix in cooled caramel, vanilla, and salt. Slowly add powdered sugar and heavy cream, beating until desired consistency is reached.

To assemble the cake, stack and frost cake layers on a cake board. Spread buttercream on the first layer, pipe a ring around the edge, and add half of the apple filling. Drizzle with caramel. Repeat with the next layer.

Chill the cake in the freezer for 5 minutes to lock in the filling. Spread a thick coat of buttercream around the cake for a textured finish.