Brush the matsutake mushrooms clean of any dirt or debris. Cut into approximately 1/8 inch thick slices. Turn each mushroom slice in the sake, then dredge with the rice flour and shake off the excess.
Heat the cooking oil over medium high heat in a deep heavy-bottomed pot. The oil should be approximately 1/4 to 1/2 inch deep. Drop a crumb of rice flour into the pot to test for heat. When it sizzles, the oil is sufficiently hot.
Cook the flour-coated mushroom slices in batches, slipping a few pieces into the hot oil, one at a time. Do not over crowd the pan. Turn each piece as it browns. When both sides are golden brown & crisp, remove them to a paper towel-lined pan and keep warm in a 200 degree oven.
Serve immediately while hot with ponzu sauce for dipping on the side.
2 Tbsp yuzu juice (substitute lime juice if yuzu is unavailable)
2 Tbsp rice vinegar
2 Tbsp light soy sauce
Mix all three ingredients well and serve in small dishes for dipping.