Special equipment: Spirograph® drawing tools, graduated round cookie cutters, three 6-inch round or two 8-inch round baking pans, a cake leveler (optional), a large piping bag, an offset spatula and a bench scraper
For the Spirograph® decor: Knead the fondant until smooth and pliable. If the fondant is sticky, knead in a little confectioners' sugar until the fondant is elastic and does not cling to your hands. Roll out the fondant between 2 sheets of parchment paper to 1/4-inch thickness using a rolling pin. Remove the top sheet of parchment paper and allow the fondant to stand until the surface is firm and dry. This can take 2 to 4 hours (and up to overnight), depending on the surrounding humidity of the room in which you're working. The fondant can be tested for readiness by attempting a Spirograph® design. Place the Spirograph® tools on top of the fondant and begin to draw using light pressure with a food color marker. If the tip of the pen sinks into the fondant, it is not dry enough to use and needs to stand longer. If the pen glides across the surface, then it's ready to use.
Use the Spirograph® tools to practice drawing on a piece of copy paper before you begin drawing on the fondant. Choose your favorite designs to replicate on the fondant.
Place the Spirograph® tools on the dry fondant and use the food color pens to draw designs onto its surface. Make various sizes and patterns. You'll need about 14 designs to cover the entire cake. Allow the drawings to dry for 10 minutes, then use the cookie cutters to stamp out the designs. Store the stamped-out circles in a single layer between 2 pieces of plastic wrap.
For the chocolate cake: Preheat the oven to 350 degrees F. Spray three 6-inch round baking pans (or two 8-inch pans) with flour-based baking spray.
Beat the butter and granulated sugar together in the bowl of an electric mixer. Add the oil; beat in the eggs 1 at a time, mixing well after each addition.
Whisk together the cocoa powder, flour, baking soda and salt in a large mixing bowl. Add the flour mixture to the creamed butter mixture alternately with the sour cream, beginning and ending with the flour mixture. Add the hot coffee at the end and mix on low speed until just combined. Use a spatula to scrape down the bowl and mix again on low until the batter is smooth.
Divide the batter evenly among the prepared pans, filling each pan no more than three-quarters full. Bake for 30 to 35 minutes, or until the cakes spring back when pressed in their centers. Turn the cakes out onto wire racks to cool. When the cakes are completely cool, level the tops using a large serrated knife or cake leveler
For the buttercream frosting: Cream together the butter and confectioners' sugar on low speed in the bowl of an electric mixer. When the mixture is combined, increase the speed to high and whip until light in color, about 3 minutes. Turn the mixer off and add the vanilla extract and 1/3 cup heavy cream. Beat again until the frosting is light and fluffy, about another 3 minutes. Add additional heavy cream if the mixture is too thick to spread or pipe.
Place 1 cup of buttercream in a medium bowl. Drape a damp towel across the bowl to prevent the frosting from drying. Set aside.
Place the remaining frosting in a large piping bag. Use the buttercream to fill the cake layers and to apply a thin crumb coat to the outside of the cake. Use an offset spatula or bench scraper to even the frosting. Place the cake in the refrigerator to chill until the frosting is firm, about 30 minutes. Meanwhile, divide the reserved 1 cup of frosting into 3 parts in separate bowls and tint each portion with one of the 3 gel food colors.
When the crumb coat layer is well chilled, apply a second layer of white frosting and even the edges of the cake with a bench scraper. Use an offset spatula to randomly place dabs of pink, orange and lavender tinted buttercream over the edges and top of the cake. Carefully scrape away some of the tinted frosting using a single motion with a bench scraper. The colors will smear and blend together, creating a watercolor effect. Add more color dabs of frosting and repeat the process as desired. Transfer the cake to the refrigerator to chill.
When the cake is well chilled, apply the fondant Spirograph® designs to the sides of the cake using buttercream as an adhesive. Use leftover white buttercream to pipe small mounds of frosting on top of the cake; cover the buttercream with assorted sprinkles and nonpareils. Place 1 fondant Spirograph® design on top of the cake if desired.