1. Make rhubarb layer: Combine rhubarb, sugar, and water in a saucepan. Simmer 10-12 minutes until softened but still maintaining some texture. Add cornstarch slurry and cook 1-2 minutes until thickened. Stir in salt, vanilla, and cinnamon if using. Cool completely, then refrigerate until chilled.
2. Prepare coconut pastry cream: Heat coconut milk, sugar, and vanilla bean (if using) until simmering. In a separate bowl, whisk egg yolks, cornstarch, and salt. Gradually add hot coconut milk to egg mixture, whisking constantly. Return mixture to saucepan and cook, whisking continuously, until thickened (4-5 minutes). Remove from heat; stir in butter, vanilla extract (if not using bean), and shredded coconut. Strain through fine-mesh sieve, cover surface directly with plastic wrap, and chill at least 2 hours.
3. Assemble pie: Spread chilled rhubarb layer evenly in baked pie crust. Top with coconut pastry cream, smoothing with an offset spatula. Cover and refrigerate at least 4 hours or overnight.
4. Finish pie: Just before serving, whip heavy cream with powdered sugar and vanilla until stiff peaks form. Top pie with whipped cream and sprinkle with toasted coconut.
5. Serve: Cut with a knife dipped in hot water and wiped clean between slices for the cleanest presentation.

Notes:

• For best results, prepare the pie a day ahead to allow flavors to meld completely.
• Frozen rhubarb can be substituted; use directly from frozen without thawing.
• Store leftover pie covered in the refrigerator for up to 3 days.
• For extra coconut flavor, use a coconut cookie crust instead of traditional pie crust.

Nutrition (Per Slice): Calories: 425, Fat: 24g, Carbohydrates: 48g, Protein: 5g, Fiber: 2g, Sodium: 220mg