This made a fantastic birthday cake.
Preheat oven to 350 degrees F. Prepare two 8-inch round cake pans with either baking spray or line with parchment.
Mix brown sugar and cinnamon together in a bowl. Set aside.
Beat butter and sugar together in a bowl of stand mixer fitted with paddle attachment 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.
Sift flour and baking powder together in a bowl then stir into creamed butter mixture by hand. Mix milk into batter until smooth.
You should have about 6 cups of batter. Pour 1 1/2 cups of the batter (or 1/4 of your total batter) into one of the prepared cake pans; add 1/4 of the apples and 1/4 of the brown sugar mixture. Lightly pat apple mixture into batter. Gently pour another 1 1/2 cups of batter over apple layer; top with another 1/4 of the apples and brown sugar mixture. Lightly pat apples into batter.
Repeat this with remaining batter and brown sugar/apple mixture in other cake pan. Bake in the preheated 350°F oven until a toothpick inserted in the center of the cake comes out MOSTLY clean, 25 to 35 minutes. (Do NOT overbake! Cake is done when the edges are golden brown and the cake has just slightly pulled away from the sides of the pan)
When ready to assemble cake prepare Cinnamon Buttercream.
Add butter, vanilla, cinnamon, confectioners sugar and 1 tablespoon in heavy cream to stand mixer fitted with paddle attachment. (You can also use a hand held mixer.)
Turn mixer on slow to incorporate all ingredients. If frosting seems too thick add remaining heavy cream.
Turn mixer on high and mix for 1-2 minutes or until frosting is lighter and fluffier. (You want frosting to be a spreadable consistency.)