In a small bowl, place the tablespoon of cold water, then sprinkle the agar-agar in a layer over top. Allow it to sit for a few minutes while you complete the next step.
In a medium saucepan, place the coconut milk, sugar and pistacho butter. Whisk together and heat until everything is melted together and steaming, but don't allow to boil.
Pour a couple of tablespoons of the hot coconut milk into the bowl with the agar-agar, and stir well. Add it slowly back into the pot, whisking the whole time. Heat for another 5 minutes, until the milk is steaming, but don’t let it boil. Whisk in the orange flower water at the end.
Divide between 4 ramekins. Refrigerate until set.
To unmold, remove from the fridge and place the ramekin in a hot water bath for a few minutes. Run an offset spatula or a butter knife around the edges of the panna cotta. Place a plate on top of the panna cotta and invert. It should slide out onto the plate. Garnish with flower petals and additional pistachios.