Add milk to a small saucepan and heat over medium heat until just boiling.

Lower heat and slowly whisk in flour, egg yolks, sugar, vanilla, and salt.

Cook on low heat, whisking constantly, until thick and bubbling (5–10 minutes).

Strain custard through a mesh sieve to remove any lumps and let cool.

Transfer custard to a piping bag and pipe 8 small dollops onto a lined tray. Freeze for 1 hour.

In a microwave-safe bowl, combine sweet rice flour, sugar, salt, water, and a few drops of food coloring. Cover loosely with plastic wrap.

Microwave for 1 minute, uncover and stir.

Continue microwaving in 1-minute intervals, stirring between each, until mochi becomes translucent and sticky (3–4 minutes total).

Let cool slightly until safe to handle.

Dust a work surface generously with potato starch. Turn mochi out and dust the top.

Shape into a 1/2-inch thick disk and divide into 8 equal pieces.

Dust hands with starch, roll each piece into a ball, then flatten into a disk.

Place one frozen custard dollop in the center and gently pinch mochi closed around the filling.