Whisk together the oil and confectioners sugar until well combined and glossy. Whisk in the egg yolk and the rosewater.
Add the rice flour, about ½ cup at a time, and the cardamon and mix well. Use your hands as needed to press together. Cover and refrigerate for at least 6 hours, or overnight, to help the flavors to mingle.
When ready to bake, preheat the oven to 350F/175C. Take small handfuls of the dough and press together into a slightly flattened ball. Place the cookies on a baking sheet (lined is probably better but they don't really stick so not too drastic).
Lightly score the tops of the cookies in a cross shape either with the tip of a knife or the edge of a spoon to be more rounded. Be careful not to cut in too far or the cookies will split wen you bake.
Top with a little crushed pistachios or poppy seeds, if you like, then bake for approx 20 minutes until still not browned but they look more dry. Allow to cool before moving.
Be aware rose waters can vary quite a lot in strength from brand to brand, so I would suggest you start with just a small amount and test the flavor before adding more in case yours is stronger.