Preheat oven to 350°F. Grease a standard Donut pan.
Combine dry ingredients (flour, sugar, green tea powder, baking powder, baking soda, salt) in a large bowl.
In a small bowl, whisk together egg, sour cream, milk and vanilla. Pour wet ingredients into dry ingredients and whisk until just combined. Stir in melted butter.
Transfer ingredients to a piping bag and pipe into molds, filling ¾ of the way up. Baked on the center rack of the oven for 10-14 minutes, or until a tooth pick inserted into the center comes out clean. Allow to cool in the pan for 5 minutes, turn out onto a wire rack and allow to cool completely. Repeat with remaining batter.
For the glaze, whisk together all ingredients except water. Whisk in 1 tablespoon of water and continue to add 1 teaspoon of water at a time until you reach the desired consistency.
When the donuts are completely cool, dip the top half into the glaze and return them to the rack until the glaze sets. Enjoy!