For the Cake

1. Preheat the oven to 350°F. Prepare three 6-inch round baking pans, line the bottom with parchment paper, and grease the sides.
2. In a large mixing bowl, combine the sugar, vegetable oil, eggs, and vanilla extract. Beat on medium speed until well incorporated. Next, mix in the sour cream and beat until well combined.
3. In a separate bowl, sift together the dry ingredients. To the batter, alternate adding half the dry ingredients followed by half of the coffee and mix just until the flour starts to incorporate. Repeat with the remaining ingredients. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
4. Divide the batter evenly between the three pans, about 1 2/3 cups of batter per pan. Bake at 350°F for 25-32 minutes. Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. Remove from the oven and allow to cool completely.

For the Buttercream

1. Cut the butter into pieces. Using the paddle attachment, whip the butter for 5-7 minutes, scraping down the bowl occasionally. Beat until the butter is whipped to a soft and spreadable consistency. 
2. Alternate adding 3 cups of powdered sugar at a time with the liquid ingredients: vanilla extract & heavy cream. Add the salt. Ensure the powdered sugar is well combined, then and whip for 1-2 minutes before adding the next cups of powdered sugar.
3. Once all the powdered sugar has been added, increase the speed to medium-high and beat for another 2-3 minutes to whip additional air into the frosting. Then turn the mixer down to the lowest speed and beat for about 2-3 minutes, which helps eliminate some of the air bubbles. 
4. In two bowls, separate out 1 heaping cup of buttercream for each color (purple and green. For the purple, dye with a violet gel color and to darken it, add a toothpick tips’ worth of black until the desired color is reached. For the green, use electric green gel and add a toothpick tip of leaf green until the desired color is reached. Set aside.
5. To the remaining buttercream, add ½ of the black cocoa powder and beat until well combined. Add additional cocoa powder as needed to achieve a dark chocolate color. Separate out 1 ¼ cup cups of the frosting, to that add the black gel color until the desired color is reached. The remaining buttercream will be used to frost between the cake layers and for the crumb coat and it doesn’t require you to color it.

To Assemble

1. Use a cake leveler to cut the domes off the top of the cake so all 3 layers are even and flat. Place a dollop of frosting on a cake board and place the bottom layer of cake on top of that.
2. The middle of the cake is frosted with the chocolate buttercream (not colored black). Using 1 cup of frosting between each layer, spread it evenly with an offset spatula. Add the second cake layer and add the remaining buttercream. Place the third layer of cake on top.
3. Next, add a crumb coat to the entire cake. First, use an offset spatula to press any excess buttercream onto the edges of the cake. Next, fill a large piping bag and cut off about 1 inch from the bottom. Pipe a thin layer of frosting along the outside edge of the cake, filling in additional gaps between the layers, and a thin layer of buttercream on top of the cake. Then, use an offset spatula or icing scraper to wipe away the excess frosting, leaving just enough to coat the outside of the cake. Do not mix the crumb-coating frosting back into the bowl if it has cake debris in it. Refrigerate the cake for 15-30 minutes until the outside layer is chilled.

To Decorate

1. Prepare the black and white fondant squares by rolling out each color to a thin but even thickness, but not less than 1/8” thick. Use a square cookie cutter (mine was 5/8” wide) and cut the squares out (estimate about 30 squares of each color). Set aside or place in a single layer in a flat, airtight container to be used later.
2. Fill 3 small piping bags with each of the 3 buttercream colors. Starting at the bottom edge, evenly pipe 3 rows of black buttercream. Next, pipe 2 rows of green buttercream and then 2 layers (or more as needed) of the purple buttercream.
3. Next, use an icing scraper to smooth out the frosting. First, run it under very hot water and dry it thoroughly. Lightly press the scraper up against the side of the cake at a 90-degree angle, rotate the cake in a circle, applying even pressure on the scraper. Stop halfway through and clean off the scraper, run it under hot water, and dry it before starting again. Repeat these steps until the buttercream is smooth. Any gaps can be filled in with the additional frosting and rescraped. Pull the top layer of purple frosting into the middle of the cake. Allow the buttercream to set for 15 minutes.
4. If you want to add tire treads, gently place the wheels up against the edges of the cake and very gently run it through the buttercream. This works best once the buttercream forms a light crust.
5. Next, place the fondant squares along the bottom edge, alternating black and white. If you find they are not sticking, use a toothpick to gently rough up the buttercream so the fondant can stick to it.
6. Add a very thin layer of the remaining chocolate buttercream to the top and cover with Oreo crumbs then place any remaining decorations on top.