For the cake: Preheat the oven to 350° F. Grease three 8-inch pans
Chop the sweet potatoes into one inch thick pieces and throw into a food processor. Pulse the sweet potato into tiny pieces and set aside.
In a medium-sized mixing bowl, combine the vegetable oil, eggs, egg white, vanilla extract and granulated sugar. Whisk together until well combined, and mixture turns lighter in color.
Add the sour cream and mix until combined.
Slowly add the cocoa powder and beat into the batter
In a separate small bowl, combine the flour, cinnamon, salt, baking soda and baking powder and whisk together. Slowly add the dry ingredients and mix slowly with a spatula until about half of the flour is incorporated. Add the sweet potato pieces and mix only until the flour is blended, being careful not to over mix.
Pour the batter into three 8-inch pan and bake at 350°F and baking for 18 to 22 minutes. To see if the cake is done, stick a toothpick into the cake and if the toothpick comes out clean, the cake is done. Cool completely.
For the frosting: Cut the butter into pieces. Using the paddle attachment, whip the butter for 3-4 minutes, scraping down the bowl occasionally. Beat until butter looks light in color.
Add the brown sugar and cinnamon, beating until combined. 4 cups powdered sugar, one cup at a time and mix on low speed until incorporated into butter. The frosting will start to thicken.
Begin to alternate heavy whipping cream and maple syrup while alternating in additional cups of powdered sugar until all the powdered sugar. Continue beating until all ingredients are combined and then beat it at a high speed for 2 minute to whip some additional air into the frosting.
To build the cake: Even out the top of the cake by cutting off the dome using a serrated knife.
Place a dollop of frosting on the bottom of the cake plate and place the bottom layer of cake on top. The frosting will help prevent the cake from sliding.
In order to create even layers, I use a piping bag fitted with an open round tip, or simply cut off the end of the piping bag. I also highly suggest using a lazy Susan to rotate the cake as you work.
Fill the first layer of frosting by using your piping bag and pipe from the outside edge into the center. Gently spread the frosting with an offset spatula to fill in the gaps.
Place the next layer of cake on top and repeat the above instructions to fill the layer of frosting.
For the top layer of cake, place it inverted, so the bottom of the cake is actually the top, creating a nice and smooth top edge.
Proceed to add your crumb coat by adding a cup of frosting to the top of the cake and gently spread out to the edges and down the sides. Use the leftover frosting in your piping bags to fill in the cracks between the layers.
Using an offset spatula or icing smoother, wipe away the excess frosting, leaving you just enough to coat the outside of the cake. Do not mix your crumb coating frosting back into your icing if it has cake debris in it. Refrigerate the cake for 15-20 minutes to allow the crumb coat to harden.
Remove the cake from the refrigerator and proceed to the remaining frosting to the top and edges of the cake. To get a nice smooth edge, use your offset spatula and icing smoother to wipe away excess frosting.
Then take your flat angled spatula and very gently press it onto the bottom edge of the cake, rotate the lazy Susan, continuing to press the knife into the cake until it reaches the top edge. This will create the rippled effect on the sides of the cake.
Take the extra frosting and pipe florets on top of the cake.
To make the ganache, microwave the chocolate chips together with the heavy cream for 30 seconds, stirs and return to the microwave for another 30 seconds or until the chocolate is melted. Drizzle with a spoon down the sides of your cake.