Place the sago pearls in a large, heatproof bowl. Bring water to a boil and let it cool for a minute, or use a thermometer to reach 60-70°C (140-158°F).

Pour the hot water over the sago pearls.

Stir immediately with a spoon or chopsticks to ensure all pearls are submerged and not clumping.

Cover the bowl and let it soak for 30 minutes. Do not stir during this time. The sago will absorb the water and become a thick, gelatinous, semi-translucent mass with a slightly grainy texture.

While the sago soaks, prepare the purple sweet potato. Peel and slice it into 1cm thick rounds for faster steaming.

Place the slices in a steamer basket. Steam over boiling water for 15-20 minutes, or until extremely tender and easily pierced with a fork.

Transfer the steamed sweet potato to a bowl. While still hot, add the condensed milk (or sugar). Use a potato masher, fork, or the back of a spoon to mash it into a completely smooth, lump-free paste. Let it cool to room temperature.

Once the sweet potato mash is cool, divide it into 12 equal portions(about 20g each if using 250g potato). Roll each portion between your palms into a smooth, round ball. Set aside on a plate.

After 30 minutes, the sago will be ready. It should look like a sticky, cohesive dough.

Gently knead the sago mixture in the bowl for just 30 seconds to make it more uniform.

Divide the sago dough into 12 equal portions (about 30-33g each). Roll each portion into a ball. Keep your hands lightly wet to prevent sticking.

Take one sago ball. Flatten it in the palm of your hand into a disc about 3-4 inches wide.

Cup the disc in your hand to form a small "bowl."

Place one sweet potato ball in the center.

Gently bring the edges of the sago wrapper up and around the filling.

Pinch the top to seal completely, then gently roll between your palms to form a smooth ball again.

If using a mold: Press the ball into a lightly dusted mooncake or dim sum mold. Press firmly, then unmold onto a piece of parchment paper.

Prepare your steamer. Line the steamer basket with parchment paper or cabbage leaves, and cut small holes for steam.

Arrange the shaped cakes in the steamer, leaving about 1 inch of space between them as they will expand slightly.

Add cold water to the steamer pot. Place the basket in the pot.

Turn the heat to high. Once the water reaches a full, rolling boil and steam is pouring out, start timing. Steam on high heat for 25 minutes.

Do not open the lid during steaming.

After 25 minutes, turn off the heat. You can remove the cakes immediately.

The cakes will be very soft and sticky when hot. Let them cool on the parchment for at least 15-20 minutes. As they cool, the sago wrapper will firm up slightly and become beautifully translucent, revealing the purple filling inside.

Serve at room temperature for the best texture. They can be enjoyed slightly warm or cold.