Preheat the oven to 180 °C or 350 °F.
In a large bowl, combine mochiko, sugar, baking powder, and salt. Sift in matcha powder. Whisk until combine.
In another bowl, whisk egg, oil, and soymilk/milk until combine.
Pour the wet ingredients to the dry ingredients. Use a spatula and mix until everything is combined with no lumps.
Grease a 8 x 8? baking pan or line it with parchment paper.
Pour the batter into the baking pan.
Tap the pan gently to release the bubbles. Or use a toothpick to lightly swirl the batter to prevent bubbles.
Bake for 40 to 50 minutes.
Turn it upside down to remove the brownie mochi from the baking pan. Let it cool on a cooling rack.
Slice into 16 small pieces.