Pour about 1½ inches of cooking oil into a large pan and heat over medium-high until the oil reaches approximately 340°F. For reference, I used 6 cups of oil in a 12-inch pan and allowed it to heat for around 12 minutes on medium-high heat.

Carefully place the sweet potato pieces into the hot oil, frying them for 4 to 5 minutes. Stir gently with a wooden or slotted spoon to ensure even cooking, until they turn a light golden brown.

Using tongs or a slotted spoon, remove the fried sweet potato pieces from the oil and place them in a stainless steel strainer over a bowl to drain. Turn off the heat and set the potatoes aside until the syrup is ready.

Add 2 tablespoons of cooking oil to a skillet, swirling it to evenly coat the bottom.

Evenly sprinkle the sugar across the oil in the skillet.

Heat the skillet over medium heat. As the sugar begins to melt—about 5 minutes in—reduce the heat to medium-low.

Lift the skillet by its handle and gently move it around to distribute the melting sugar evenly in the oil. Avoid stirring, as this may cause crystallization. Continue moving and adjusting the skillet for another 2 to 3 minutes until all the sugar has melted into a golden-brown syrup. Then, turn off the heat.

Reheat the oil from the first fry for about 2 to 3 minutes until it reaches 340°F. Add the previously fried sweet potato pieces and fry them for an additional 1 to 2 minutes, or until they achieve a crunchy, golden-brown texture.

Using tongs or a slotted spoon, remove the sweet potato pieces from the oil and transfer them to a stainless steel strainer over a bowl to drain. Turn off the heat.

Reheat the prepared syrup, then add the fried sweet potatoes to the skillet. Use a wooden spoon to gently mix, ensuring each piece is thoroughly coated in the syrup. Sprinkle with black sesame seeds, if desired.

Remove from heat and transfer the coated sweet potatoes to a large baking sheet lined with parchment paper.

Arrange the pieces with a small space between each to prevent them from sticking together. Let them cool down completely.

Once the syrup has hardened, transfer the mattang to a serving bowl and enjoy.