In a small bowl, mix black sesame paste, honey, and softened butter until smooth. Refrigerate for 20 minutes until firm enough to roll into 12 small balls.

In a microwave-safe bowl, whisk together glutinous rice flour, sugar, and water until smooth with no lumps.

Cover bowl with plastic wrap and microwave on high for 2 minutes. Remove, stir well with a wet spatula, then microwave for another 1-2 minutes until the dough becomes translucent and sticky.

Generously dust a clean work surface with cornstarch. Transfer the hot mochi dough onto the surface and dust the top with more cornstarch.

Once cool enough to handle, divide the dough into 12 equal portions using a bench scraper or knife dusted with cornstarch.

Flatten each portion into a small disc in your palm, place a black sesame filling ball in the center, then pinch the edges together to seal completely.

Roll gently into a smooth ball and place seam-side down. Dust with additional cornstarch to prevent sticking.

Garnish each mochi with a sprinkle of black sesame seeds. Serve immediately or store in an airtight container at room temperature for up to 2 days.