Preheat the oven to 350F, and butter & flour two 6" cake pans.

In the bowl of a stand mixer, combine the flour, baking powder, baking soda, salt, sugar, and apple pie spice. Add in the butter, and beat on low until it's incorporated and has a sandy texture.

In a small bowl, whisk together the oil, eggs, sour cream, and Rodelle vanilla, and add them to the dry ingredients. Mix until combined.

Pour in the milk, and beat on medium-low until smooth and creamy. Finally, stir in the apple pieces.

Divide batter evenly among the two pans, and bake for 40-45 minutes, until a toothpick inserted in the center of the cake comes out with just a few crumbs. Let cakes cool completely before decorating.

While you wait on the cakes, prepare the brown butter streusel.

First, brown the butter. Place the ½ stick of butter in a small nonstick pot, and heat over medium. After the butter melts, it will start bubbling and foaming. Let cook, stirring occasionally, until it turns a light golden brown and starts to smell toasted and nutty. Remove immediately from heat to avoid burning. Stir in the brown sugar and apple pie spice, then add in Flahavan's oats and flour. Mixture will be a little clumpy! Set aside for now.

When ready to assemble, prepare the buttercream. In the bowl of a stand mixer fitted with the whisk attachment, beat the butter until smooth and fluffy. Add in the powdered sugar, one cup at a time, until fully incorporated. Add in some milk as needed if it's too thick. Finally, mix in the vanilla, apple pie spice, and pinch of salt.

To assemble the cake, divide the 2 cake layers in half to create 4 thin layers. Place one cake piece on your cake stand or platter, and pipe frosting all over. Smooth with an offset spatula if desired and sprinkle on some of the streusel mixture. Repeat with all.

Pipe frosting all over the outside of the cake, and smooth using a bench scraper. Decorate around the top using a large star 1M tip and sprinkle additional streusel in the center. If desired, decorate the outside with some caramel drips. Place prepared caramel in a squeeze bottle, and apply where you'd like. Serve immediately or store in fridge until needed. Enjoy!