Peel the sweet potato, cut into slices, and soak in water. (I tried to make my slices thinner than 1 cm in width.)
Boil in a pot until it becomes soft.
When it becomes soft, drain the water. Add the butter while it's still hot, and mash.
I mash until it's still a little bit lumpy, but please adjust it to your liking.
Add the soft light brown sugar, and soy milk, then mix well.
Divide into 8 portions, and shape them. Place on a baking sheet or in muffin cups, and brush their surfaces with the egg yolk.
Bake in the toaster oven for 15-20 minutes. (They brown differently depending on where they are positioned, so please switch their positions around half way through.)
You can increase the amount of soy milk (or milk) to make the batter soft, and squeeze it out from an icing bag. These are also delicious! Make them creamier by adding heavy cream.
Story Behind this Recipe
I love sweets, but store-bought ones contain a lot of sugar. I tried making some at home to keep the sugar content low.
I like the part where I can use the same pot from boiling the potatoes until mixing the ingredients. If you have sweet potatoes, you can make this with the ingredients you have at home even without butter. This is also makes things easier.