Mix egg yolk, 20g caster sugar, lemon juice, and cornstarch evenly.
Boil the milk and lemon zest, then turn off the heat. Gradually pour into the egg yolk mixture while stirring quickly.
Strain the mixture back into the pot and stir until thickened. Let it cool.
In a separate bowl, mix cream cheese and 10g caster sugar until smooth.
Add whipping cream and the cooled lemon custard sauce. Stir until smooth and well combined.
For the mochi skin, mix glutinous rice flour, cornstarch, caster sugar, and milk thoroughly. Sift the mixture, cover with plastic wrap, and poke a few small holes.
Steam the mixture over high heat for 25 minutes. While still hot, mix in the butter until uniform.
Optionally, add a drop of food coloring. Stretch the dough repeatedly until elastic.
Knead the dough into small balls, coat with cooked glutinous rice flour, and roll out into round sheets.
Place each sheet in a mold, fill with the prepared lemon cheesecream filling, seal tightly, and trim off excess dough.