Bake the sponge cake. While the cake is still warm, remove from the cake tin and place it upside down in a plastic bag. This is to make it moist with the steam that develops. Still in the bag, place in the freezer to freeze.
When the sponge hardens, cut it into about 1 cm pieces. Cut out circles with a circular mold (use a shape of about 4~5 cm in diameter). Cover with plastic wrap and refrigerate.
Remove the skin of the sweet potato. For the cream and the paste you should have a total of 500 g prepared. It's easiest to use two pots and add 200 g and 300 g consecutively.
Submerge in water and remove any scum. This will involve changing the water a few times. I change the water between giving the baby milk.
Boil the sweet potato. The 200 g portion will boil quicker so make sure they are both sufficiently soft before placing in a colander and removing any liquid. Using a blender or the like to create a smooth paste.
Add the ☆ ingredients to Step 5 and when smooth, return to the pot. Add milk in small increments while heating. Prevent from scorching by stirring continuously with a spatula.
Place the softened, boiled potato to make the cream in a colander and, as in Step 5, blend to a paste. Add the ★ ingredients and return to the pot after mixing.
Add a little of the designated milk to Step 7 and mix, then heat. Add the remaining milk and, taking care not to scorch the mixture, keep mixing until thoroughly heated.
Add sugar and vanilla extract to chilled cream and whip until stiff. Be careful not to over-whip, as the cream will form a lumpy, unpleasant texture.
On a cutting board or the like, lay out the cake from Step 2 and in the center of each, spread out a dollop of the paste. Add Step 9 to a piping bag and squeeze around the paste. The photo here shows the dome in progress.
Adjust the firmness of the sweet potato cream with cream. The photo shows 130 ml. (It was possibly a little too soft...)
Swirl the sweet potato cream twice around the edge of Step 10's sponge and then pipe the cream into the shape of your choice. If the center of the mound is higher, it'll look cuter.
After letting them firm in the freezer for a little, place the cakes into paper cups or the like. Decorate with your choice of topping and you're done. This is topped with sweetened boiled sweet potato and mint.
I decorated the tops with powdered sugar.
A cross-section looks like this.
This is a dome-shaped sweet potato Mont Blanc using a 16 cm diameter sponge cake.
Story Behind this Recipe
My parents sent me plenty of sweet potatoes so I decided to try and make an intricate cake. Quite a few sweet potatoes were used up just during the experimental phase.
The softness of the cream is important. If it's too firm, it won't pipe out well. Even if you pipe it out very carefully, if the cream is thick and lumpy the finished product will look unattractive. If you don't have a blender you can strain the mixture with a fine-mesh sieve. If you want to bake it into one large cake, shaping it into a dome.