Prepare the chocolate truffle filling: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Pour the hot cream over the chopped chocolate in a heatproof bowl and let it sit for a minute. Stir until the chocolate is fully melted and smooth. Add the butter and vanilla extract, and stir until fully incorporated. Let the ganache cool slightly before placing it in the refrigerator for about 30 minutes to firm up.

1. Prepare the mochi dough: In a heatproof bowl, whisk together the glutinous rice flour and sugar. Slowly add the water, stirring until smooth and lump-free. Transfer the bowl to a steamer or a pot with a steamer basket, cover with a damp cloth, and steam over medium heat for 20-25 minutes, stirring every 5 minutes. The mochi should become thick, glossy, and slightly translucent. Let the mochi cool for about 10 minutes before handling.
2. Form the chocolate truffles: Once the ganache has firmed up in the refrigerator, scoop out small portions (about 1 teaspoon each) and roll them into smooth balls. Set them aside.
3. Assemble the chocolate truffle mochi: Dust a clean surface with cornstarch to prevent sticking. Scoop a small amount of mochi dough and flatten it into a circle with your fingers. Make sure it’s large enough to wrap around the truffle. Place a chocolate truffle in the center of the dough, pinch the edges to seal it, and gently roll it into a ball, ensuring the chocolate is fully enclosed. Repeat with the remaining dough and truffles.
4. Serve: Dust the completed mochi with more cornstarch to prevent sticking. Serve immediately or refrigerate for later. Best enjoyed within 1-2 days while the mochi is soft and chewy.