Gonzo Cake
Author: 
Yield: 1 8-inch (20-cm) three-laYer cake • 10 to 12 servings
 
Ingredients
For the Purple Velvet Cake
  • 1 (4-ounce/115-g) package purple yam powder (about 1⁄2 cup plus 2 tablespoons; see Baked Note)
  • 1⁄4 cup (60 ml) canola oil or other vegetable oil
  • 21⁄4 cups (285 g) cake flour
  • 3⁄4 cup (90 g) all-purpose flour
  • 1 tablespoon baking powder
  • 1 teaspoon baking soda
  • 3⁄4 teaspoon kosher salt
  • 4 ounces (1 stick/115 g) unsalted butter, softened, plus more for the pans
  • 1⁄4 cup (50 g) vegetable shortening, at room temperature
  • 2 cups (400 g) granulated sugar
  • 1 tablespoon pure vanilla extract
  • Blue and red food dyes or gels
  • 4 large egg whites, at room temperature
For the Cream Cheese Frosting
  • 3 cups (340 g) confectioners’ sugar
  • 8 ounces (2 sticks/225 g) unsalted butter, softened
  • 2 (8-ounce/226-g) packages cream cheese, softened
  • 1 tablespoon plus 1 teaspoon pure vanilla extract (optional)
  • 1⁄2 teaspoon kosher salt
For Décor
  • Purple sprinkles (optional)
Instructions
Make the Purple Velvet Cake
  1. Preheat the oven to 325°F (165°C). Butter three 8-inch (20-cm) cake pans, line them with parchment paper, and butter the parchment. Dust with flour and knock out the excess.
  2. In a small saucepan over very low heat, stir together 2 cups (480 ml) water with the purple yam powder and cook, stirring occasionally, until the mixture is rehydrated, between 5 and 20 minutes depending on the heat. Once the mixture looks and feels like mashed potatoes (or mashed yams) remove it from the heat and whisk in the canola oil.
  3. In a large bowl, sift both flours, the baking powder, baking soda, and salt. Set aside.
  4. In the bowl of a standing mixer fitted with a paddle attachment, beat the butter and shortening on medium speed until creamy, 3 to 4 minutes. Add the sugar and vanilla and beat on medium speed until the mixture is fluffy, about 3 minutes. Turn the mixer to low. Add the flour mixture in three equal parts, alternating with the purple yam mixture, beginning and ending with the flour mixture. Scrape down the sides of the bowl with a rubber spatula and then mix on low speed for a few more seconds.
  5. Mix equal drops of red and blue food dyes in a small bowl to make purple, then scrape it into the cake batter and mix until a pale purple color is achieved.
  6. In a clean bowl, whisk the egg whites by hand or in your standing mixer until soft peaks form; do not overbeat. Using a rubber spatula, gently fold the egg whites into the batter.
  7. Divide the batter equally among all three pans. Use your spatula to spread the batter evenly. Bake the cakes, rotating the pans halfway through baking, until a toothpick inserted in the center of the cakes comes out clean, 20 to 30 minutes. Transfer the pans to a wire rack and cool for 20 minutes. Invert the cakes onto the rack, remove the pans, and let them cool completely. Remove the parchment.
Make the Cream Cheese Frosting
  1. Sift the confectioners’ sugar into a large bowl and set aside.
  2. In the bowl of a standing mixer fitted with the paddle attachment, beat the butter until it is completely smooth. Add the cream cheese and beat until combined.
  3. Add the confectioners’ sugar, vanilla, if using, and salt and beat just until smooth; do not overbeat or the frosting will lose structure. Chill the frosting in the refrigerator for about 5 minutes. (The frosting can be made up to 24 hours ahead; cover the bowl tightly, refrigerate, and let the filling soften at room temperature before using.)
Assemble the Gonzo Cake
  1. Place one cake layer on a serving platter. Trim the top to create a flat surface and evenly spread about 11⁄4 cups (330 g) of the frosting on top. Add the next layer, trim and frost it, then add the third layer. Trim the top layer. Spread a very thin layer of frosting over the sides and top of the cake (a crumb coat, which helps to keep loose cake crumbs under control when you frost the outside of the cake) and place it in the refrigerator to firm up, about 15 minutes.
  2. Remove the cake from the refrigerator. Frost the sides and top with the remaining frosting. Sprinkle the outer edge of the top of the cake with the sprinkles, if you like. Chill the cake in the refrigerator to set the frosting, about 15 minutes. Slice and serve.
Notes
How to store: This cake can be covered in a cake saver in a cool room for up to 3 days. If you refrigerate it, make sure to cover it tightly and bring it back to room temperature before serving.

Substituting Sweet Potatoes for Yam Powder:
- Swap the yam powder in the recipe for 2 cups (420 g) roasted, peeled, and mashed sweet potatoes. Ideally, use fresh sweet potatoes, though canned puree without any other ingredients should be okay.
- Bake the mashed sweet potato cake layers slightly longer than the yam powder layers, 5 to 7 more minutes.
- Omit the purple food dye. the mashed sweet potato cake is a pretty sherbet-orange color, and the purple dye will just ruin it.
Recipe by Baked Sunday Mornings at http://bakedsundaymornings.com/2015/07/06/in-the-oven-gonzo-cake/