Whisk rice flour, cornstarch, salt, and turmeric in a medium bowl to combine. Whisk in 1½ cups of the hottest water from your tap (aim for about 120°) followed by coconut milk. Let batter sit, uncovered, until thickened to the consistency of half-and-half (it will be slightly gritty from the flour), 30–45 minutes. If batter is too thick, whisk in about 1 Tbsp. water to get it to the right consistency.