In a large bowl, combine glutinous rice flour, cornstarch, sugar, and strawberry powder. Mix well to remove any lumps. Gradually add the milk while whisking until smooth and fluid. Strain the batter once for an extra-silky texture, then cover tightly with plastic wrap.

Steam the batter over medium heat for 20 minutes until fully cooked with no raw center. While still hot, add the butter and stir until melted. Using gloved hands, knead and stretch the dough for 3–5 minutes until smooth, elastic, and evenly colored. Cover and let cool to room temperature.

Beat the softened cream cheese (and sugar, if using) until completely smooth. Add the strawberry jam and mix well. In a separate bowl, whip the cold heavy cream to soft peaks. Gently fold it into the strawberry cream cheese mixture in two additions. Transfer to a piping bag and refrigerate.

Dust your work surface with cooked glutinous rice flour. Divide the cooled mochi dough into 8–10 equal portions (25–30g each). Roll each into a ball and flatten into an 8–9cm round, keeping the edges thinner than the center.

Place a wrapper into a mold, pipe 30–35g filling in the center, then gently gather and pinch to seal. Trim excess dough if needed. Flip seam-side down and gently shape.

Dust lightly with strawberry powder or top with a strawberry slice. Chill the mochi for 1–2 hours before serving for the best texture and flavor.