Make the Mochi Dough: In a large, heatproof bowl, whisk together 120g glutinous rice flour, cornstarch, and 20g sugar. Gradually pour in the milk while whisking until you have a perfectly smooth, lump-free batter. Strain the batter through a sieve back into the bowl for extra smoothness. Cover the bowl tightly with plastic wrap.

Steam the Dough: Place the bowl in a steamer over boiling water. Steam over medium heat for 20 minutes, or until the dough is fully set with no liquid center. Carefully remove the bowl. While the dough is scorching hot, add the butter (and food coloring/pumpkin powder, if using). Use a spatula to roughly mix, then once cool enough to handle, put on gloves and knead, stretch, and fold the dough repeatedly for about 5 minutes until it's smooth, elastic, and evenly colored. Cover with plastic wrap and let it cool to room temperature.

Toast the Flour & Prep: While the dough cools, make the toasted flour (熟粉, shou fen). In a dry pan over low heat, cook 30g glutinous rice flour for 3-5 minutes, stirring until lightly aromatic and pale gold. Set aside to cool. Whip the chilled heavy cream with 20g sugar until stiff peaks form. Transfer to a piping bag and refrigerate.

Shape & Fill: On a clean surface dusted with toasted flour, roll the cooled dough into a log. Divide it into 6-8 equal pieces. Take one piece, roll it into a ball, and flatten it into a disc. Generously dust both sides with toasted flour and roll it out into a thin circle about 9cm in diameter. Drape the skin over your 5cm mold or a small cup. Gently press it in to line the mold.

Assemble the Mochi: Pipe a little cream into the bottom. Place one peeled mandarin in the center, then pipe more cream over and around it to encase the fruit. Gather the edges of the mochi skin, pinch them together firmly at the top, and twist off any excess dough. Dust the pinched seal with toasted flour.

Chill & Serve: Place the mochi pinched-seam-side down on a plate. Optionally, decorate the top with a tiny dot of cream or a small piece of orange peel. Refrigerate for at least 1-2 hours before serving to allow the flavors to meld and the texture to set perfectly.