Mix rice flour, 1 cup coconut, sugar, and baking powder together in a bowl. Whisk milk, oil, eggs, and vanilla extract together in a separate bowl; stir into flour mixture until well mixed. Fold red bean paste into rice mixture and pour into a 9x13-inch baking dish.
Bake in the preheated oven for 1 hour. Cool at room temperature for 15 minutes before serving. Garnish with about 1 tablespoon coconut.