Steam the taro pieces until completely soft, about 20 minutes. Mash until smooth, then mix with 1/4 cup sugar, condensed milk, butter, and vanilla if using. Refrigerate the filling for at least 30 minutes until firm enough to shape.

Divide the chilled taro filling into 8 equal portions and roll each into a smooth ball. Return to the refrigerator while you prepare the mochi dough.

In a heatproof bowl, whisk together the sweet rice flour and 1/4 cup sugar. Add the water and stir until completely smooth with no lumps remaining.

Cover the bowl with plastic wrap and steam over high heat for 15 minutes until the dough becomes translucent and stretchy. The texture should be glossy and elastic.

Dust a clean work surface generously with potato starch. Transfer the hot mochi dough onto the surface and dust the top heavily with more starch.

Once cool enough to handle (about 2 minutes), divide the dough into 8 equal pieces. Coat your hands with starch before touching the dough.

Flatten one piece of dough into a 4-inch circle, keeping the edges slightly thinner than the center. Place a taro ball in the middle and gather the edges up and around the filling, pinching firmly to seal.

Roll the sealed mochi in potato starch to coat completely, shake off excess, and place seam-side down. Repeat with remaining dough and filling. Serve immediately or store in an airtight container.