Rinse the glutinous rice under cold water until the water runs clear, then soak in cold water for 4 hours or overnight. Drain completely.

Steam the soaked rice in a steamer basket lined with cheesecloth for 25-30 minutes until translucent and tender. While hot, transfer to a bowl and stir in 1/2 cup coconut milk, 2 tablespoons sugar, and salt until absorbed. Let cool to room temperature.

In a microwave-safe bowl, whisk together sweet rice flour, 1/4 cup sugar, and 3/4 cup water until smooth. Cover with plastic wrap and microwave on high for 2 minutes, then stir. Microwave for another 1 minute until the mixture becomes translucent and stretchy.

Dust a clean work surface generously with toasted sweet rice flour or cornstarch. Turn out the hot mochi dough and knead for 1-2 minutes until smooth, dusting with more flour if sticky. Let cool for 5 minutes until warm but workable.

Divide the mochi dough into 8 equal pieces. Flatten each piece into a 4-inch circle using your palm or a rolling pin, keeping the edges slightly thinner than the center.

Place 1 tablespoon of coconut sticky rice in the center of each circle, top with 3-4 cubes of fresh mango, then fold up the edges around the filling like a dumpling.

Pinch and twist the seams tightly to seal, then gently roll between your palms to smooth the surface. Place seam-side down on a parchment-lined plate.

Refrigerate the mochi for at least 30 minutes to firm up before serving. Just before serving, warm the remaining 1/2 cup coconut milk and drizzle lightly over each mochi if desired.