Preheat your oven to 350ºF and position the racks in the oven at the 1/3 and 2/3 positions. This allows for even baking of all three cake layers. Butter and line the bottoms of three 8-inch round cake pans with parchment paper, ensuring that the parchment is also buttered. This will prevent the cakes from sticking.
In a large mixing bowl or stand mixer, whisk together the granulated sugar, flour, cocoa powder, baking soda, baking powder, salt, cinnamon, cloves, and nutmeg. This dry mixture is essential for the flavor of the cake. Make sure everything is well combined.
In a separate bowl, whisk the sweet potato puree with the eggs until the mixture is smooth. Then, add in the vegetable oil and vanilla extract, mixing until everything is well incorporated.
Make a well in the center of the dry ingredients and pour in the sweet potato mixture along with buttermilk. Mix on low speed until the dry ingredients are just incorporated, being careful not to overmix. You want a smooth batter, so scrape down the sides of the bowl as needed.
Divide the batter evenly among the prepared cake pans, using approximately 2 cups of batter per pan. Bake in the preheated oven for 30 to 40 minutes, rotating the pans halfway through baking. A toothpick inserted into the center should come out clean when the cakes are done.
Once baked, place the pans on wire racks and allow the cakes to cool to room temperature. After cooling, run a knife around the edges of each pan, then invert onto a wire rack. Carefully remove the parchment paper and let the cakes cool completely.
To prepare the ganache, place the finely chopped chocolate in a heat-proof bowl. In a small saucepan, warm the heavy cream over medium-low heat until it just starts to steam—do not let it boil. Pour the warm cream over the chocolate and let it sit for 30 seconds. Then, whisk gently until the chocolate melts and the ganache is smooth.
Incorporate the unsalted butter into the ganache a little at a time, whisking until it’s fully incorporated. Refrigerate for 30 to 40 minutes or until the ganache thickens and becomes spreadable.
For the buttercream, combine egg whites, granulated sugar, and 2 tablespoons of molasses in a heat-proof bowl set over simmering water. Whisk gently until the mixture is warm and the sugar is dissolved, about 140ºF. Transfer to a stand mixer fitted with a whisk attachment. Beat on medium-high speed until the meringue is completely cool—this can take up to 45 minutes.
Once cooled, switch to the paddle attachment and add the unsalted butter, one tablespoon at a time, mixing well after each addition. Increase the speed to high and beat until the buttercream is light and fluffy. Mix in the remaining molasses and vanilla extract to taste. If not using immediately, cover and refrigerate until needed.
To assemble, level each cake layer by cutting off the domed tops with a serrated knife. Place one layer on a cake stand or serving plate. Top it with 1/2 cup of buttercream and spread it evenly to the edges. Pour half of the ganache over this layer, smoothing it out.
Place the second layer on top and repeat with another 1/2 cup of buttercream and the remaining ganache. Finally, place the third layer on top, flat side up.
Cover the entire cake with a thin layer of buttercream to create a crumb coat. This helps to keep any stray crumbs in place. Refrigerate for about 15 minutes to set the crumb coat, then spread a thicker final layer of buttercream over the top and sides. You can use the extra buttercream to pipe decorative details on the cake as desired.
Optionally, top the cake with meringue cookies and a light dusting of cocoa powder before serving at room temperature. Enjoy!
