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.

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.