Preheat your oven to 325°F (165°C).
Grease 6 cups in a muffin tin with room temperature unsalted butter. Set aside.
In a bowl, combine the glutinous rice flour, matcha powder, baking powder, and salt. Whisk together until well mixed.
In a separate medium mixing bowl or 1-quart measuring cup, whisk together the milk, egg, honey, melted butter, and vanilla extract.
Add the dry ingredients to the wet ingredients and whisk until smooth. The consistency should be thick but runny, similar to syrup. If needed, add more milk, a tablespoon at a time, to achieve the proper consistency.
Divide the matcha mixture equally among the 6 prepared muffin cups.
Bake for 25-30 minutes, until the tops are set and slightly golden.
Let the mochi bites cool in the pan for 10 minutes before carefully removing them to a cooling rack to cool completely. The mochi bites will puff up in the oven but will sink slightly as they cool.
Optionally, dust with icing sugar or drizzle with melted chocolate before serving.
Store the mochi bites in a sealed container at room temperature for 2-3 days for best results.