Peel the skins from the sweet potatoes, slice into thin rounds, then boil until tender.
Drain the hot water, mash while still hot, add butter, then mix well. Add milk, sugar, and salt, then simmer on low heat.
Stir with a wooden spoon to prevent the batter from sticking to the bottom of the pan, and cook out the liquid. Remove from heat once the batter becomes thick and heavy.
Let the cakes cool to the touch, then thoroughly whisk the egg yolk. Shape the cakes into boat-like shapes, then arrange on a parchment paper-lined baking sheet.
For the egg wash, mix a small amount of water to the egg white, paint it on the surface of the cakes, then brown the cakes in the toaster oven.
Story Behind this Recipe
I wanted to come up with a sweet potato cake recipe that I could savor the rustic deliciousness of the sweet potato. By adding 1 teaspoon rum to the ingredients, you can turn this into a delightful dessert for "grown-ups." Add rum in Step 4 with the egg yolk.
Allow the liquid of the sweet potato to cook out, to make a thick, malleable batter that is tasty and shapes easily. The batter burns easily, so don't take your eyes off of the pot. If you paint the egg wash twice while baking, the cakes will turn out even nicer looking.