In a small bowl, mix the brown sugar and the cinnamon. Set aside.
Preheat the oven to 325 degrees. Prepare three 8-inch, or four 6-inch round cake pans. Spray them with cooking spray, line with parchment paper, and spray again. Set aside.
In a medium sized bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, oil, and sugar on high until fluffy and smooth, about 4 minutes.
Add the eggs, one at a time, to the bowl and beat on medium until well incorporated. Scrape down the sides after each addition. Add the vanilla extract and beat on medium speed for another minute.
Turn the mixer on low and add ½ of the flour mixture to the butter mixture and mix until fully incorporated. Then add half of the buttermilk and blend well, followed by the rest of the flour mixture, and the rest of the buttermilk, mixing on low. Don’t over mix, you only want to mix just until incorporated.
Pour about 10 ounces of the batter in each 8-inch pan or 8 ounces in each 6-inch pans. Divide the cinnamon/brown sugar mixture between the cakes, sprinkling evenly over the batter, then top with the remaining batter (about 8 oz. more per 8-inch pan or 6 oz in each 6-inch pan).
With a small offset spatula, or a spoon (I tried a spoon, but found more success with a small offset spatula) evenly distribute the cake batter so it’s even on top.
Bake for 30-33 minutes, or until an inserted tooth pick comes out mostly clean.
Remove cakes from the oven and let them cool in pans for 15 minutes before taking them out of the pan and placing them on cooling racks.
When cakes are completely cooled, wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until you’re ready to frost. If you are a beginner, I recommend putting them the freezer because it is so much easier to frost a slightly frozen cake.
For the cinnamon dulce de Leche buttercream
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes. Add the dulce de Leche and beat for 2 more minutes.
With the mixer on low speed, gradually add the powdered sugar. When it's all combined, add the heavy whipping cream, cinnamon, salt and vanilla. Turn the mixer to medium-high speed and beat for about 5 minutes.
Using a wooden spoon, beat frosting to get the air bubbles out.
Assembly
If the cakes need to be leveled, level them. Place a dollop of frosting in the middle of a cake board (the frosting acts as a glue to hold the cake to the board). Place a cake layer, topside up on the cake board.
Spread ½ cup of the dulce de Leche buttercream evenly over the cake layer. Then add ⅓ cup of the canned dulce de Leche right on top of the buttercream.
Place a new cake layer, topside down onto the buttercream/dulce de Leche and repeat the same steps by adding the buttercream and then add the canned dulce de Leche.
Apply a thin coat of frosting around the entire cake to create a crumb coat. Chill the crumb coat in the freezer for 10 minutes to lock in crumbs.
After the crumb coat is chilled, continue frosting the cake.