Grease a 9×13 inch pyrex with butter. Get a large mixing bowl and stir together the dry ingredients.
In a blender, add the milk and vanilla. Stir in the peanut butter and blend until you get peanut butter milk.
Add the peanut butter milk to the dry mixture and mix thoroughly. Pour into the pan. Cover tightly with foil. Place the pan in the center of the over and bake for 45 minutes. Uncover and cool for several hours.
When cool, use a plastic knife to cut the peanut butter mochi. Roll each piece around in some potato starch to keep from sticking to each other.