[Preparation]
Soften the butter at room temperature.
Prepare room temperature egg.
Preheat the oven to 200℃ / 392 F.

[For Sweet Potato Filling]
Place the diced sweet potatoes, granulated sugar, and water in a pot and heat over medium heat. Heat while stirring occasionally.
When the water is almost gone and it has thickened, turn off the heat.

Add salt and butter, and mix gently to coat the potatoes. Take it out in a small bowl and put the bowl in cold water to cool it well. Set it aside.

[For Cake Batter ]
Cream unsalted butter until smooth. Add cane sugar in 3 parts and mix well each time. Use an electric hand mixer to beat it on low speed for 2-3 minutes until whitish.

Put an egg in another small bowl and beat it lightly until watery.
Add the egg to the mixture in 4 parts. Beat it until well combined and smooth each time. The mixture should become like a smooth cream.
Note: Use a room temperature egg to prevent separation. If it's cold, warm it up a little in a hot water bath.

Add vanilla oil and mix well. Add ground almonds. Use a spatula to mix until smooth while crushing lumps.

Sift half of the dry ingredients (cake flour and baking powder) over the mixture and fold it 10 times.
Add half of the soy milk and fold it 10 times.

Sift the remaining dry ingredients over the mixture and fold it 10 times.
Add the remaining soy milk and fold it until you don’t see flour left.

Add 2/3 of the sweet potato filling (not hot) and fold roughly.
Put the batter in glassine-paper-lined muffin pan cups. Top with the remaining sweet potato filling and press lightly.

Lower the oven to 180℃ / 356 F and bake for 30 minutes until golden brown.
Take it out of the oven and let it cool for 5-10 minutes, then take it out from the pan and let it cool on a rack. If you don't eat it right away, wrap it with cling wrap while it's slightly warm.