Preheat oven to 375 degrees F. Grease the bottom and sides of the cake pans liberally with nonstick spray.
Prepare the cake batter: Combine the eggs and sugar in the bowl of an electric mixer and beat on high speed until light and lemon-colored, 5-8 minutes. Meanwhile, whisk the mayonnaise and water together in a medium bowl; set aside. Finely chop 1/2 cup walnuts and combine with the flour, baking powder, baking soda, cocoa powder, coffee and salt in a medium bowl. Stir 1/2 of the flour mixture and 1/2 of the mayonnaise mixture into the eggs with the mixer on low until well blended. Do not over mix. The less you mix flour, the more tender the cake! Scrape down the bowl and repeat with the remaining flour and mayonnaise mixtures.
Bake the cake: Fill each cake pan with 1/3 of the batter. Place the pans in the center of the oven. Rotate the pans halfway through cooking. Bake until the center is firm or when the tip of a small knife emerges clean when it pierces the center of each cake, about 16 minutes. Remove from the oven and let cool for 1/2 hour before unmolding. Cool completely before frosting.
Candy the oranges: Cut the orange in half lengthwise (from the stem to navel ends). Continue cutting the orange pieces in half lengthwise until you have 16 wedges. Place the orange wedges in a skillet and fill with enough cold water to cover. Simmer over medium heat for 5 minutes. Drain the water and repeat this step to make the peels tender and remove bitterness. Gently remove the wedges from the pan and set aside. Place the sugar, 2 cups water, the cloves and the orange peels into the skillet. Cook over medium heat until the sugar dissolves and return the orange wedges to the skillet. Continue cooking over low heat until the syrup thickens and the wedges become translucent, about 45 to 60 minutes. Transfer the wedges to a baking sheet to cool.
Make the frosting: Place the chocolate in a medium bowl and the cream in a small pot to warm over medium heat. When the cream simmers, pour it over the chocolate and let it sit for a few minutes, then whisk gently until the chocolate blends with the cream. Cool completely. Blend the confectioner's sugar, butter and vanilla in the bowl of a food processor until smooth, 5-8 minutes. Stir the cooled chocolate into the butter and sugar mixture until smooth.
Frost the cake: Chop 1 cup of walnuts; set aside. Place one cake round, top side down, onto a serving platter. Top with 1/4 of the frosting letting some of it spill over the top to coat the sides. Sprinkle 1/4 cup of the walnuts on top of the frosting. Repeat with the next cake layer. Place the final layer on top of the other two, bottom side up, then coat the top and sides of the cake with the remaining frosting. Top with the remaining 1/4 cup of walnuts and the candied orange wedges.
Prepare the cake using a cake mix with 1/2 cup finely chopped walnuts added to the batter before baking in place of the scratch cake, if desired.