Preheat oven to 275 degrees F.
Grease a 9″ x 13″ pan (or an 8" x 8" pan, for a half-recipe).
Mix the mochiko flour, sugar, cocoa, salt, and baking powder together.
In a separate bowl, mix the vanilla, water, and coconut milk together.
Blend the wet and dry ingredients thoroughly until completely smooth.
Pour the mixture into the greased pan.
Cover tightly and bake for 90 minutes.
Remove the pan from the oven.
Remove the cover and allow to cool completely (which will probably take several hours).
Once cooled, replace the cover. Can be stored at room temperature for up to 3 days.