Rinse the sweet potatoes and cover them in a sheet of newspaper. Wet the potatoes with the newspaper, squeeze lightly, and cover with aluminum foil.
Bake the potatoes for 70 minutes in a 250℃ preheated oven. Test them with a skewer and make sure you can poke through easily. When they're cooked well enough, peel them.
Place the peeled sweet potatoes into a food processor. Add the butter and sugar while the potatoes are still hot, and switch the food processor on. Blend them well.
Add the heavy cream into the mixture from Step 3. Also add the egg yolks one by one and blend well. Then add the rum, and blend everything again.
Transfer the mixture from Step 4 into a piping bag and squeeze it into the pudding cups. Combine and mix the egg yolk and mirin (for the glaze) and brush it onto the sweetened sweet potatoes.
Bake in 250℃ oven for 10 minutes to finish.
These are delicious just-cooked. If they have cooled down, cover them with aluminum foil to bake lightly in a toaster oven.
If you're shaping them with your hands rather than squeezing them with a piping bag, cover the mixture from Step 4 with plastic wrap, press out the air, and chill in the fridge. (It's easier to shape when it's chilled.)
You can make roughly 10 of these in a 12 cm × 5 cm boat-shaped aluminum cups.
Also, you can simply pour or scoop the mixture into the pudding cups without using piping bags.
Story Behind this Recipe
Since I like my sweetened sweet potatoes to be smooth, fine, and rich, I used quality ingredients and made them with care.
Sweet potatoes are fantastic when they're baked over long time. It might be troublesome, but I highly recommend you use the oven to bake the potatoes rather than using a microwave. *If you don't own a food processor, please mash your sweet potatoes and use a strainer.