Preheat oven to 350 degrees F. Lightly grease donut pan and set aside.
In a large bowl, whisk together flour, baking powder, salt, matcha powder, and sugar. In another bowl combine egg, melted butter, sour cream, and milk. Add wet ingredients to dry ingredients. Stir until there are no longer any dry streaks of flour.
Transfer batter to a piping bag fitted with a large round tip. Fill greased donut pan about 2/3 full of batter. Bake for 13-15 minutes until donuts have puffed up. Allow doughnuts to cool in pan for 2 minutes before unmolding. Allow cake doughnuts to cool to room temperature on a wire rack.
To make matcha glaze: whisk together confectioners' sugar, milk, and matcha powder. Dip cooled doughnuts into matcha glaze. Place dipped doughnuts on a wire rack and immediately sprinkle black sesame seeds. Allow glaze to set. Once top glaze is set, invert doughnuts to allow bottom to set.