three 6-inch cake pans; three 9-inch cake pans; 6-inch cardboard round; 4 long cake dowels or bubble tea straws; edible pen; bag with 2D tip; 1 long sharpened cake dowel
For the cake: Preheat the oven to 350 degrees F. Grease the bottom and sides of three 6-inch cake pans with cooking spray. Grease the bottom and sides of three 9-inch cake pans with cooking spray.
For the 6-inch cakes: Place 1 box cake mix, 1 cup water, 1/3 cup vegetable oil and 3 eggs in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed for 30 seconds. Increase the speed to medium for 2 minutes. Divide the batter among the prepared 6-inch pans and bake until a toothpick inserted in the center comes out clean, about 30 minutes. Cool completely on a wire rack.
For the 9-inch cakes: Place the remaining 2 boxes cake mix, 2 cups water, 2/3 cup vegetable oil and 6 eggs in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed for 30 seconds. Increase the speed to medium for 2 minutes. Divide the batter among the prepared 9-inch pans and bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes. Cool completely on a wire rack.
For the buttercream: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on high speed until fluffy, about 2 minutes. With the mixer on low speed, gradually add the confectioners' sugar and milk, 1 tablespoon at a time, alternating between the two until no lumps remain. Add the vanilla and salt and mix until incorporated. Transfer the buttercream to a piping bag fitted with a 2D tip.
Place one 9-inch cake layer on a 10-inch cardboard round and place on a rotating cake stand. Pipe 1 to 1 1/2 cups buttercream on top and use an offset spatula to spread into an even layer. Place a second 9-inch cake layer on top of the buttercream and pipe another 1 1/2 cups buttercream on top. Spread into an even layer, then top with the last 9-inch cake layer.
To apply a crumb coat, pipe about 2 cups buttercream on the top and sides of the cake and use an offset spatula or bench scraper to make smooth. Remove excess buttercream and scrape into a bowl. Transfer cake to the refrigerator and chill for 30 minutes.
Repeat the process with the 6-inch cakes and chill.
Remove the cakes from the refrigerator.
Press a 6-inch cardboard round in the center of the 9-inch cake to make an outline. Remove the cardboard. Push 1 cake dowel or bubble tea straw into the cake within the outline you just made.
Make a line with an edible pen where the top of the cake hits the dowel. Remove the dowel and cut at the line. Use the cut dowel as a model to cut 3 more. Push the 4 dowels evenly into the cake within the outline.
Gently place the 6-inch cake on top of the dowels. Thread a long dowel through both cakes. Cover the top of the wooden dowel with frosting.
Using a small round tip, pipe more buttercream in the gap between the cakes, or smooth with a spatula.