In a mortar and pestle or spice grinder, grind rose petals with sugar until they become a fine powder.
1. Preheat oven to 325 F.
2. Cream butter until light, fluffy, and very soft - it should be the consistency of mayonnaise. Add salt and rosewater and stir to combine well.
3. Add rose sugar and powdered sugar; cream with butter until thoroughly combined.
4. Sift together flour and brown rice flour; stir into butter mixture just until combined.
5. Pat dough evenly into a buttered 8 x 8" pan and smooth the top.
6. Bake until starting to brown at the edges; middle should still be light, about 22-28 minutes.
7. Allow to cool briefly on a wire rack.
8. While still slightly warm, sprinkle superfine sugar over the top of the shortbread, tilting pan to coat evenly or spreading with your fingers. Use a thin knife to slice cookies into desired size. I cut about 1" x 2.5", which is seven cookies by three cookies, for a total of 21.
9. Allow to cool completely.
Keep cookies in an airtight container for up to 2 weeks.