Preheat oven to 350°F/177°C.
Prepare a 6-cavity doughnut pan (if using non-stick, grease with a little oil).
In a medium bowl, mash the banana, egg, sugar, vanilla and miso paste together until combined.
Add in the milk, vegetable oil and give it a stir.
Next, add in the baking powder, kinako and cake flour.
Fold the mixture with a spatula until just combined.
Transfer the mixture into the doughnut pan, filling each cavity until almost full.
Give the pan a few taps to release any trapped air bubbles.
Bake at 350°F/177°C for 13-15 minutes, until a toothpick inserted comes out clean or with a few crumbs clinging to it.
Let the doughnuts cool in the pan for 5 minutes before transferring to a wire rack to cool completely.