To make rose hip decoction, bring water and rose hips to a low boil in a medium saucepan. Reduce heat and simmer for 20 minutes. After 20 minutes, remove from heat and add rose petals, stirring to combine (the petals only nee to steep for a few minutes until the color and aromatics have diffused into the rose hip decoction).

Strain rose decoction into a large pyrex liquid measuring cup. It should measure four cups. Add more water or pour out to adjust accordingly.

Add 2 cups of rose decoction, lemon juice, and sugar to a heavy bottomed saucepan. Bring to a gentle boil and continue to heat without stirring until the sugar mixture reaches 240 degrees on a candy thermometer (adjusting heat on burner to keep the mixture from bubbling over or burning). Remove from heat.

Meanwhile, mix remaining rose decoction, corn starch and cream of tartar in a small saucepan. Heat over medium high heat until mixture becomes extremely thick and almost paste like. Remove from heat.

Once the sugar mixture and the cornstarch mixture are ready, carefully combine the two in the bowl of a stand mixer fitted with a whisk attachment, or by careful whisking the two together. CAUTION: The sugar mixture will be very hot. If your mixture remains lumpy, pulse it in a blender until the mixture is smooth.

Prepare a 13x9 baking dish by lightly greasing with butter or coconut oil, lining with plastic warp, then grease plastic wrap as well. Then sift one tablespoon cornstarch and one tablespoon powdered sugar into baking dish, rotating dish to spread powder throughout pan. Set aside.

Return mixture to a heavy bottomed, medium saucepan. Over low to medium heat, continue to cook this mixture for 1.5-2 hours, stirring frequently. Carefully monitor temperature to ensure the candy does not scorch. The mixture will become very thick and almost stiff. As the mixture reaches this point, you will need to stir constantly to prevent it from burning. It is ready when a spoon drawn through the bottom of the pan separates the mixture and spoonful of mixture dropped in ice water remains very firm and holds its shape. At this point add the rose water and optional pistachios. You may need to cook the mixture just a little longer to drive off the moisture introduced by the rose water.

Remove from heat and pour into prepared dish. Smooth with a silicone spatula and pat into corners as the mixture will not likely spread very easily if you have cooked it long enough. Cool at room temperature for one hour, and then chill in refrigerator for at least another hour. Cut into squares and roll in equal parts cornstarch and powdered sugar that have been sifted together. IF, at this point, the candy is too soft to hold its shape return to saucepan and cook until until the mixture is stiffer, testing in ice water again.

Store in an airtight container in cool, dry place to avoid candies sticking together.