How to Make It
Preheat oven to 375°.
To prepare donuts, weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, granulated sugar, matcha, baking powder, baking soda, and salt in a large bowl, stirring with a whisk.
Combine 1/2 cup buttermilk, water, butter, 1 teaspoon vanilla, and egg, stirring with a whisk. Make a well in center of flour mixture; add buttermilk mixture, stirring until well combined.
Coat a 12-capacity donut pan with cooking spray. Spoon batter into a large zip-top plastic bag. Snip a small hole in 1 bottom corner of bag; pipe batter evenly into pan (to form 12 donuts). Bake at 375° for 12 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack for 5 minutes. Carefully remove donuts from pan; cool completely on wire rack.
To prepare glaze, combine powdered sugar, 2 tablespoons buttermilk, and 1/2 teaspoon vanilla (add additional buttermilk if glaze is too thick). Working with 1 donut at a time, dip donut down into glaze; flip donut over, and place on rack. Repeat with remaining donuts. Allow glaze to set before serving.