Put the marshmallows and the milk in a heat-proof bowl.
Microwave for 1 minute. Melt 90% of the marshmallow by microwave and leave the rest. If the marshmallow is melted completely in the microwave, the marshmallow will turn hard.
Take out the bowl from the microwave and mix well until all of the marshmallows have melted.
Slowly add the hot water to the matcha and dissolve it.
Add a little amount of the marshmallow into the matcha and mix (this makes sure that the mixtures don't separate).
Pour the matcha into the rest of the marshmallow.
Pour the double cream into the bowl and mix well.
Cool the matcha mix with ice pack / ice cubes.
After the matcha mix is a thick texture, pour it in small glasses.
Keep them in the fridge until it hardens.
Put whipped cream and anko (aduki bean paste) for decoration.
Notes
If you pour the marshmallow+matcha mixture into small glasses before it turns into a thick texture, the two mixtures will separate, so make sure you wait for the mixture to cool a little bit.