Mix the dough base: In a large bowl, whisk together the glutinous rice flour, sugar, and pumpkin puree until evenly combined and the mixture looks crumbly.

Add water gradually: Stream in warm water a little at a time, stirring with a spoon or your hands until a shaggy dough forms. Stop adding water once the dough holds together without being sticky.

Knead the dough: Knead the dough on a clean surface for about 3 minutes, until smooth, slightly tacky, and no longer cracking at the edges.

Divide into portions: Divide the dough into 12 equal pieces, about 1 1/2 tablespoons each, and roll each piece into a smooth ball.

Flatten and fill: Flatten each ball into a disk about 2 1/2 inches wide, place a scant teaspoon of red bean paste in the center, and gather the edges up to seal completely.

Roll into balls: Roll each filled piece gently between your palms to smooth out the seam and shape into a round ball.

Score pumpkin ridges: Using the dull edge of a knife or a chopstick, press 6 to 8 evenly spaced lines into the sides of each ball to create pumpkin grooves, pressing in about halfway without cutting through.

Add the stem: Press a short piece of cinnamon stick into the top center of each dumpling to look like a pumpkin stem, tucking a mint leaf beside it if using.

Arrange in the steamer: Line a steamer basket with parchment paper and arrange the dumplings with at least 1 inch of space between them so they don't stick together.

Steam until glossy: Steam over medium heat for 12 to 15 minutes, until the dough turns glossy and springs back when pressed gently with a finger.

Brush with oil: Remove the dumplings from the steamer and brush lightly with toasted sesame oil while still warm to add shine and prevent drying out.

Cool slightly and serve: Let the dumplings cool for 5 minutes before serving, as the filling stays hot and the texture firms up slightly as it cools.