Step 1Heat the oven to 350 degrees. In a bowl, stir together the flour with either the rice flour or potato flour and salt.
Step 2In another bowl, beat together the butter and powdered sugar until well combined. Add the vanilla, orange flower water (if using) and the dry ingredients, mixing until blended. The dough will be very soft.
Step 3Transfer the dough to the center of a round, 9-inch tart pan with a removable bottom. With lightly floured fingertips or the back of a lightly floured spoon, press the dough into the pan, working from the center to the edges to create a thin, even layer. Using the sharp tip of a paring knife, score the dough into 12 wedges. Sprinkle evenly with the fine sugar.
Step 4Bake for 25 minutes in the lower third of the oven. The shortbread should be set when lightly touched and lightly browned.
Step 5Cool the pan on a rack for 5 to 10 minutes. Following the score marks, cut the shortbread into slices. Cool completely before carefully removing from pan. Store in an airtight container for up to 3 days.