Preheat oven to 350° degrees F. Spray two 8-inch round cake pans with baking spray or line each with parchment paper.
In a medium bowl, mix together brown sugar and cinnamon. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar until smooth and creamy (1-2 minutes). Beat in eggs, 1 at a time, until incorporated.
Add vanilla extract. Once incorporated remove bowl from mixer.
In a bowl, whisk together flour and baking powder. Then, stir it into the creamed butter mixture by hand. Mix milk into batter until smooth.
You should have about 6 cups of batter. Pour 1½ cups of the batter (or ¼ of your total batter) into one of the prepared cake pans. Then, add ¼ of the apples and ¼ of the brown sugar mixture. Lightly pat the apple mixture into the batter. Gently pour another 1½ cups of batter over the apple layer. Top with another ¼ of the apples and brown sugar mixture. Lightly pat apples into the batter.
Repeat this layering with the remaining batter and brown sugar and apple mixture in the second cake pan.
Bake for 25-35 minutes, or until an inserted toothpick comes out mostly clean. (Do NOT overbake!) The cake is done when the edges are golden brown and the cake has just slightly pulled away from the sides of the pan.
Let the cakes cool before assembling and adding the cinnamon buttercream.