Preheat the oven to 350°F and line the bottoms of three 6” cake pans with parchment rounds. Set aside.
In a medium bowl, stir together flour, cinnamon, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, use an electric mixer to beat together melted butter, oil, granulated sugar, brown sugar, eggs, greek yogurt, and vanilla extract on low-medium speed until creamy and smooth.
With the mixer on low speed, mix in the flour mixture until just combined. Slowly pour in the milk with the mixer still on low speed until just combined and no lumps remain.
Divide the batter into the prepared cake pans, approximately 320g of batter in each pan. Bang the cake pans on the counter a couple of times to release any large air bubbles.
Bake cakes for 35-40 minutes or until a toothpick inserted in the middle comes out clean. The edges should be lightly golden brown and the middles are set.
Let cool on a wire rack for at least 20 minutes before turning the pans upside down onto a sheet of parchment or wire rack to release the cakes. Peel off the parchment rounds and let cake layers cool completely before leveling and assembling.
Salted Caramel Sauce
In a small pot, add in the granulated sugar and let it sit on medium-high heat undisturbed until it completely melts and turns an amber colour.
With one hand, very, very slowly pour the heavy cream into the caramelized sugar while stirring constantly and vigorously with a spatula in the other hand. If the sugar crystallizes (if the cream was too cold or you added it too fast), just keep heating it and stirring it until it melts again.
Cook caramel for another few minutes until it coats the back of a spoon. Remove pot from heat and stir in the butter, one cube at a time, until all combined.
Keep the caramel in the pot or transfer to another container to cool. The caramel will continue to thicken significantly as it cools.
Caramel Buttercream Frosting
In a large mixing bowl, use an electric mixer or stand mixer with a paddle attachment to beat softened butter on medium-high speed for 10 minutes until it becomes very creamy and very pale yellow in colour.
Add in the powdered sugar and mix on low speed until combined. Add in the caramel and continue to beat on medium-high speed for another 10 minutes until creamy and fluffy.
Use a spatula to smear the buttercream around the bowl to release some of the air bubbles before using.
Assembling the Cake
Place one cake layer down on a cake turntable. Add a large dollop of buttercream on top and use an offset spatula to smooth it into an even layer. Use the spatula to create a rough and shallow circle in the middle of the frosting.
Add a heaping spoonful of caramel to the middle of the layer and spread evenly, staying 1” away from the edges of the cake. Repeat with the rest of the layers.
Frost the cake with a thin crumb cake layer. Chill cake in the fridge for about 15 minutes before frosting the cake with a final layer of buttercream. Chill the cake again for 15 minutes.
Measure out about 80-100g of caramel for the drip coating. Test the consistency of the caramel by putting a dollop on the side of a bowl and watching how it drips. If it’s too runny, let it chill for longer. If it’s not dripping enough, heat it in 5-10 second intervals in the microwave.
Pour all the caramel on top of the cake. Use a clean offset spatula to push the caramel to the edges of the cake and let it drip over the edge. When the caramel reaches your desired level of drippiness, let it chill in the fridge while you prepare the apple topping.
In a small pot, add in apples, brown sugar, and cinnamon. Cover and cook on medium-high heat until it comes to a boil. When the apples are tender but not mushy, sprinkle in cornstarch and stir until the liquid turns into a jelly-like consistency.
Transfer apples to a bowl and chill in the fridge completely or until it’s barely warm to the touch, about 30 minutes. Arrange apple cubes on top of the cake as desired.