Whisk the rice flour and coconut in a small bowl. Beat the butter, 1/2 cup confectioners' sugar, the salt and vanilla seeds in a large bowl with a mixer on medium speed until smooth and creamy, about 2 minutes. Beat in the egg, then beat in the flour mixture until just combined. Cover and refrigerate until the dough is no longer sticky, about 1 hour.
Preheat the oven to 325 degrees F. Roll heaping tablespoonfuls of dough into balls and arrange 1 inch apart on a baking sheet. Refrigerate until firm, about 15 minutes.
Bake the cookies until set and lightly browned on the bottom, about 20 minutes. Let cool 10 minutes on the baking sheet.
Put the remaining 3/4 cup confectioners' sugar in a small bowl. Gently roll the warm cookies in the confectioners' sugar and return to the baking sheet. Reroll the cookies in the confectioners' sugar, then transfer to a rack to cool completely.