In a mixing bowl, combine mochiko (sweet rice flour) with sugar. Gradually add water, whisking until smooth, and let it sit for 5 minutes.

Transfer the mochi mixture into a microwave-safe bowl, cover loosely with plastic wrap. Microwave for 1 minute, stir, and repeat for a total of 4-5 minutes until thick and slightly translucent.

Carefully remove dough and stir in vanilla extract. Set aside to cool slightly.

Dust a workspace with cornstarch, pour dough onto it, and spread into a rectangle, cooling for 10-15 minutes.

Divide the cooled dough into 6 pieces, flatten each, and spoon crushed Oreos into the center.

Fold edges over filling, pinch to seal, and dust with cornstarch to prevent stickiness.

Place filled mochi into an airtight container, dusting with cornstarch, and refrigerate for at least 30 minutes.

Serve and optionally drizzle with melted chocolate or pair with matcha tea.