Rinse the sweet rice under cold water until it runs clear, drain water completely, then add it to the rice cooker with the water, pandan extract, and optional pandan leaves. Give it a quick stir to ensure panda extract is diffused evenly. Turn on the rice cooker using either “normal” or “sweet rice” setting and once it's done, crack the lid and let the rice cool down until slightly warm.

In a microwave safe bowl, whisk together the glutinous rice flour, sugar, corn starch, milk, oil and ube extract. Cover with a damp paper towel and microwave for 2-3 minutes, stirring every minute or so, until it turns into a thick, bouncy dough. Let it cool to room temperature.

In a medium pot, combine the coconut milk, sugar and salt. Cook over medium low heat, stirring constantly, until the sugar and salt dissolve but don't let it reach a boil. Turn off the heat and set aside ½ cup of the coconut sauce. Stir the cornstarch slurry into the remaining sauce and cook over low heat, stirring constantly, until it thickens slightly.

Once the rice has cooled down, pour the reserved ½ cup of coconut sauce over it. Mix well so the rice soaks it up then let it sit for about 10 minutes to absorb.

Turn the mochi out onto plastic wrap or parchment paper and shape into a ball. Cut it in half with a plastic spatula, then top with another sheet of plastic wrap or parchment and roll each half into a thin layer with a rolling pin.

With a piece of flattened ube mochi, add a portion of the pandan sticky rice and 1-2 mango pieces, then fold the mochi over until it's sealed shut. Dust the mochi with corn starch, brushing off any excess so it's evenly coated on both sides.

Serve with the remaining coconut sauce on the side.