Preheat the oven to 350°F. Line a baking sheet with parchment paper.
In a small saucepan on medium heat, gently stir the milk, sugar and vegetable oil until it is steaming but not boiling.
Whisk in the flour until there are no clumps of dry flour remaining.
Remove the saucepan from the heat. Use a whisk or a spatula to mix in the tapioca flour and the matcha powder.
Crack in the egg and continue mixing until the batter is completely smooth.
Add the batter into a piping bag and pipe 8 blobs of batter in a circle on your prepared baking sheet.Make sure the bubbles are touching slightly. Otherwise you'll just end up with little blobs of donuts, instead of connected rings.
Bake for 15 to 20 minutes, until the donuts are dry and solid on the outside. Let cool and enjoy plain. Or decorate with melted chocolate and sprinkles, glaze, or icing.
Notes
Use a piping bag with a large round nozzle to create the little bubbles of dough in rings. If you don't have a piping bag, fill a large Ziplock bag with the donut mixture and cut one of the bottom corners off to use that to pipe the dough.
If the dough balls aren't sticking together, just wet the sides with a little water, which will help glue them together.
Dip your finger in cold water and gently press down on the dough bubbles to make them perfectly round and smooth.
Allow your baked mochi donuts to cool completely, then store them at room temperature in an airtight container for up to 3 days.