Ok, the post title is just an abbreviation of what the correct name of this cake should be. In essence it is a sweet potato layer cake, with toasted marshmallow buttercream, brown sugar and cinnamon swiss meringue buttercream and candied walnuts.
Yes wow, a mouthful! And I am not going to lie, a cake with that many components does take a while. But my oh my was it worth it!
This cake was made for my boyfriend's sisters birthday and I believe she loved it as much as we did. The sweet potato makes for a dense yet fluffy cake, complimented perfectly by the toasted marshmallow buttercream. The walnuts add another texture to the cake, a very satisfying crunch and the warming spices in the Swiss meringue buttercream perfectly finish the cake off.
Sweet Potato and Marshmallow Layer Cake
Adapted from Sweetapolita
Serves at least 10
For the Cake Layers:3 large sweet potatoes (about 900 g)
2 cups sugar
4 large eggs, at room temperature
1 1/4 cups vegetable oil
2 cups plain flour
2 tbsp ground cinnamon
2 tsp baking powder
1 tsp salt
1 tsp ground nutmeg
1 tbsp ground ginger
2 tbsp white port (you could also use brandy)
2 teaspoons vanilla extract
3/4 cup non-crystallized ginger, chopped finely
For the Filling:
16 large marshmallows (The pink and white variety)
1 cup icing sugar
230g butter, at room temperature
1/2 tsp vanilla extract
1 jar (213 g) Marshmallow Fluff (For Australians, Thomas Dux sells this)
For the Swiss Meringue Buttercream:
5 large egg whites
1 1/4 cups light brown sugar
340g butter, softened and cut into cubes
2 tsp vanilla extract
pinch of salt
2 tbsp cinnamon
For the Walnuts
30g unsalted butter
2 tbsp brown sugar
2 cups walnuts, broken into small pieces
1. Preheat oven to 180 degrees Celcius (350F). Grease two 8-inch round springform pans and line the bottoms with baking papaer
2. Place the sweet potatoes on a microwave-safe plate and pierce them with a fork. Microwave until they are tender (about 7-8 minutes each side). Carefully remove the skin when cool enough to touch, and mash the flesh into a coarse puree.
3. In the bowl of an electric mixer fitted with the paddle attachment, beat the sugar and eggs together on medium-high speed until light and fluffy, about 5 minutes. Add the oil and beat on medium until combined. Add the cooled sweet potato puree and mix until combined.
4. Sift dry ingredients together (flour, cinnamon, baking powder, salt, nutmeg and ground ginger) and then add to sweet potato mixture. Mix on the lowest speed of your electric mixture until just combined. Add the white port, vanilla and non-crystalised ginger, again mixing on the lowest speed until just combined.
5. Evenly distribute batter into the prepared pans (weigh them if possible with digital kitchen scale to make sure the mixture is equally shared), smooth with a spatula and place in the center of the middle rack of the oven, about 2 inches apart. Bake for 40 minues or until a knife or skewer inserted into the center comes out clean. Let pans cool on a wire rack 10 minutes. Invert cakes onto rack and leave until they have cooled completely.
For the Marshmallow Filling:
6. Place marshmallows on a baking tray lined with baking paper. Place the marshmallows under the grill until nice and brown on top, between 30-60 seconds. Remove pan and gently turn the marshmallows over, and grill until they are golden brown. (Be sure to keep an eye on them--they burn very quickly.)
7. In an electric mixer fitted with paddle attachment, combine butter and icing sugar on low until blended, about 1 minute. Add vanilla and mix on med-high for about 3 minutes. Add marshmallow cream and toasted marshmallows, and mix on lowest setting for about 1 minute. Spoon mixture into another bowl until needed.
For the Swiss Meringue Buttercream
8. Wipe the bowl of an electric mixer with paper towel and lemon juice, to remove any trace of grease. Add egg whites and sugar to a heavy based saucepan and heat over very low heat, whisking constantly, until the sugar has completely dissolved and the egg whites are hot to touch.
9. Transfer the mixture to a stand alone electric mixture. With whisk attachment of mixer, begin to whip until the meringue is thick, glossy, and the bottom of the bowl feels neutral to the touch (this can take up to 10 minutes). Switch over to paddle attachment and, with mixer on low speed, add butter cubes, one at a time, until incorporated, and mix until it has reached a silky smooth texture (if the mixture separates like it can easily do, refer to the bottom of this post for a solution). Add vanilla and salt, continuing to beat on low speed until well combined. Add cinnamon to taste and blend.
For the Walnuts
10. Melt the butter in a small pan. Mix in brown sugar, and add the walnuts. Toss to coat.
Cook on low heat for about 7 minutes, stirring occasionally. Be sure to watch carefully so they don’t burn.
11. Turn out onto baking paper and let cool. Once cool separate the mixture in half. Place one half in a ziplock bag, removing the air, and crushing gently with a mallet until they become a fine crumb. Set this bag aside as it is for outside decoration of the cake.
12. Slice both cake layers in half horizontally, so you have 4 cake layers. Place the first layer on a plate, pedestal or cake board cut side up (so bottom of the cake layer is touching plate), and spread 3/4 cup of Toasted Marshmallow Filling over it. Sprinkle with a handful of candied pecans pieces.
Repeat previous step until you get to the final cake layer.
13. Frost the outside of the cake with the Brown Sugar Buttercream. Chill the cake for at least 30 minutes so the icing can harden up. Once the buttercream is harder and not too soft, use your hands to press the finely crumbed walnuts around the sides of the cake to decorate. Finish the cake by piling some of the non-crystallized ginger on top of the cake.
Say happy birthday and devour.
Katie and William embracing their sibling love
Notes: Finished cake can be kept at room temperature for up to 8 hours. Keep refrigerated if longer than 8 hours, but always serve at room temperature (Swiss Buttercream should never be served cold, as it goes back to a cold-butter texture). This cake will take about 4 hours to come to room temperature once taken out of the fridge.
If your Swiss Meringue Buttercream separates when adding the butter don't fret. Simply remove about a third of the mixture into a microwave safe bowl, and zap in the microwave until smooth and melted- about 30 seconds. With the electric mixture on medium, slowly add the melted buttercream to the bowl, as you would oil to a mayonnaise mixture. Sure enough your buttercream will come back together and form the luscious divinity that it is.