Thaw frozen puff pastry sheets according to package directions — approximately 30–40 minutes at room temperature or overnight in refrigerator — pastry should be pliable but still cold to the touch — if sheets become too warm and soft during preparation return to refrigerator for 10 minutes before proceeding.

Preheat oven to 400°F and allow to fully preheat before pastries go in — full preheat is essential for proper layer separation.

Place pistachios in food processor and pulse until finely chopped into small irregular pieces — not ground to powder — alternatively place in zip-lock bag and crush with rolling pin to similar consistency.

Transfer chopped pistachios to a bowl and add granulated sugar, ground cinnamon, and melted cooled butter — mix well until pistachios are evenly coated and sugar and cinnamon are distributed throughout — mixture should be moist and clumping slightly.

Lay one thawed cold puff pastry sheet flat on a lightly floured surface or parchment-lined baking sheet.

Spread pistachio filling evenly across entire surface of the pastry leaving a small border around all edges.

Lay second puff pastry sheet directly on top of the filling pressing gently to adhere and create a firm even sandwich.

Using a sharp knife cut assembled pastry into diamond or square shapes — cut all the way through both layers completely — do not leave any partially cut sections.

Whisk together egg white and water until combined into a smooth thin wash.

Using a pastry brush apply a thin even layer of egg wash over the entire top surface of cut pastry pieces only — do not apply egg wash to the cut sides as it glues layers together and prevents separation during baking.

Place baking sheet on center rack of fully preheated 400°F oven and bake for 30–35 minutes until deeply golden brown visibly puffed with distinct flaky layers and cooked all the way through.

If making optional honey syrup — combine ¼ cup water and ¼ cup honey in a small saucepan during the last 10 minutes of baking — bring to a boil then reduce to a simmer for 2–3 minutes until slightly thickened — set aside.

Remove from oven immediately and drizzle generously with pure honey or prepared honey syrup while pastries are still hot from the oven — residual heat helps honey soak into top layers.

Allow to cool for 5–10 minutes until honey glaze sets and filling firms up slightly before serving.

Serve warm or at room temperature.