In a large bowl, whisk together cake mix, whole milk, sour cream, butter, eggs, cinnamon, and vanilla.

Divide batter evenly into two greased or parchment paper-lined 9-inch round cake pans.

Bake at 350 degrees for 20-25 minutes, or until a toothpick inserted near the center comes out clean.

Cool for 15 minutes before removing cakes from pans to wire racks.

While cakes are still warm, gently poke holes with a fork, toothpick, or similar utensil, but not poking through the bottom of the cake.

In a small bowl, whisk together sweetened condensed milk, butter, brown sugar, and cinnamon.

Divide the sweetened condensed milk mixture between the two layers, drizzling it evenly over the top, keeping it away from the edges by about 1/2-inch. Use a knife or spoon to keep the mixture on the top of the cakes, and not down the sides. Allow the cakes to sit and absorb the mixture for about 5-10 minutes.

Refrigerate cakes til firm, about 1 hour. This will make transferring them to a serving platter much easier. They become slightly fragile with all the holes.

Meanwhile, prepare snickerdoodle buttercream.

In a large mixing bowl, combine butter, 4 c. powdered sugar, cinnamon, vanilla, and cream.

Beat on medium speed for 2 minutes.

Add more powdered sugar or cream as needed to achieve desired consistency. It should be thick and fluffy, but able to be piped.

Beat an additional 3-5 minutes for fluffiness.

Carefully transfer one cake to a serving platter. Spread it with a 1/4-inch thick layer of buttercream. Arrange the second cake layer on top.

Frost cake. Cut and serve.