Toss peeled & diced apples in 1 Tbsp light brown sugar and 1 tsp cinnamon. Set aside.
Apple Spice Cake:
Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, combine flour, baking powder, spices, and salt. Set aside.
In the bowl of a stand mixer combine vegetable oil and sugars. Beat until well combined (2 mins). Add eggs one at a time, fully incorporating after each addition. Add vanilla.
Add flour mixture in 3 parts. Fold in apples.
Spread batter evenly into prepared pans and bake for 45-50 mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Place sugar and water into a medium pot, stir to combine, but to not stir from this point forward. Cook over high heat, washing down the sides of the pot with a pastry brush dipped in water as needed to prevent crystals.
Cook until desired color of caramel is reached (amber) and immediately remove from heat. Very slowly pour in heavy cream while whisking quickly. The mixture will bubble up (a lot) and boil. Add butter, return to heat, and bring back to a boil. Cook for 2 minutes whisking constantly.
Remove from heat and let cool to room temperature. Transfer to a container and place in fridge to thicken.*
Caramel Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.**
Place bowl over a hot water bath on the stove and whisk constantly until the mixture reads 160F on a candy thermometer and is no longer grainy to the touch (approx. 3mins).
Place bowl on your stand mixer and whip on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.
Add cooled caramel and whip until smooth.***
Trim cake layers if needed. Place one layer of cake on a cake stand or serving plate. Top with 2/3 cup of frosting, spread evenly. Repeat with remaining layers.
Frost and smooth the outside with a thin crumb coat. Chill for 20mins.
Using a small spoon, place dollops of caramel around the top edges of the cooled cake, allowing some to drip down. Fill in the top of the cake with more caramel and spread evenly with an offset spatula.****
* The caramel can be made the day before and left in the fridge overnight. ** Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen. *** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. **** If caramel has thickened too much, microwave in 5-10 second intervals until it's pourable (but not warm).