In a microwave-safe mug (at least 12 ounces), sift together the flour, matcha powder, sugar, baking powder, and salt. Use a fork to ensure no lumps remain, especially with the matcha.
Add the milk, oil, and vanilla extract to the mug. Whisk vigorously with a fork or small whisk for 30 seconds until the batter is completely smooth and no flour pockets remain at the bottom.
Tap the mug gently on the counter 2-3 times to release any air bubbles. Smooth the top of the batter with the back of a spoon.
Microwave on high power for 60-90 seconds, depending on your microwave wattage. The cake is done when the top looks set and springs back lightly when touched, but the center may still look slightly glossy.
Let the mug rest for 30 seconds to finish cooking through with residual heat. The cake will pull away slightly from the sides of the mug.
Top with whipped cream, fresh berries, a sprinkle of matcha powder, or enjoy it straight from the mug with a spoon.