Preheat the oven to 350°F/177°C.
Dry ingredientsIn a medium bowl, whisk together the mochiko flour, brown sugar, baking powder, cinnamon, and salt. Wet ingredientsIn another medium bowl, whisk together the milk, egg, melted butter, vanilla extract until combined.
Combine wet and dry ingredients. Mix until combined.
Transfer the donut batter into a piping bag fitted with a large round tip. If you do not have a piping bag, you can use a spoon to scoop the batter.
Grease your donut pan using cooking spray or butter. Pipe or spoon the batter into each cavity, filling it all the way to the top. Once each cavity is filled, you can use the back of a spoon or knife to smooth the tops.
Bake for 30-35 minutes. Cool donuts in the pan for about 5 minutes, then transfer them onto a wire rack and let them cool completely before adding the cinnamon sugar.