1. Prepare the Crust:

1. Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper and draw a 9-inch circle as a guide.
2. In a saucepan, bring water and butter to a boil. Add flour all at once and stir vigorously until a smooth dough forms and pulls away from the pan. Remove from heat and let cool slightly.
3. Beat in eggs one at a time until fully incorporated and dough is glossy. Transfer dough to a piping bag and pipe along the drawn circle to form a ring.
4. Bake for 30-35 minutes until puffed and golden. Turn off oven, prick puffs with a knife to release steam, and let cool inside with the door ajar for 10 minutes. Then cool completely on a rack.
5. Whip heavy cream with powdered sugar until stiff peaks form. Fold in pistachio paste until smooth. Slice the cooled puff ring horizontally and fill with pistachio cream.
6. For the glaze, whisk powdered sugar, milk, and rosewater until smooth. Drizzle over the filled ring and garnish with chopped pistachios. Serve immediately or chill until ready to serve.