Peel the sweet potatoes with a peeler then cut into 5mm thick round slices.
Put into a bowl with water. Refresh the water 2-3 times, then strain in a colander.
Separate the egg yolk from the egg white.
Put the sweet potatoes into a heatproof container (like a plastic steamer), add 2 tablespoons of water, then close the lid or cover with saran wrap.
Put into the microwave and warm for 4 minutes at 600 watts. Remove from the microwave, uncover, add the butter, mix, then cover and microwave for another minute.
Uncover and add the egg white and 1/2 of the egg yolk. Mash with a potato masher or fork.
Different varieties of sweet potato have varying amounts of moisture. If the potato doesn't become smooth after mashing, add a small amount of milk to reduce the thickness.
Separate the sweet potatoes into foil baking cups, then line them up on an oven toaster tray.
Take the leftover egg yolk and brush a little over each sweet potato cup using a spoon or brush.
For starters, bake for 5 minutes in an oven toaster, then check the appearance of the potatoes. Continue baking while watching them closely until they brown to a nice color.
Once the potatoes start to brown, they will darken very quickly. Keep watching them so they don't burn!
Here's another dessert recipe using sweet potato "Sweet Potato Style! Sweet Potato Cheese Cake" (Recipe ID: 2384843)
"Sweet Potato Toast" (Recipe ID: 2379464)
"Sweet Potato Cake in a Frying Pan" (Recipe ID: 1999109)
Story Behind this Recipe
This is an arrangement of a recipe I made before called "Sweet Potatoes with a Lekue Steamer." In this version I increased the amount of butter. Also I added half an egg yolk into the batter so there wouldn't be any leftover.
When you bake these in an oven toaster, the potatoes may brown differently depending on where they are on the tray. If you want them all to bake evenly, try moving the cups around while they are baking.