Heat the oven to 180°C (350°F) and line a baking tray. Peel the bananas and cut a shallow channel along the length of each one, scooping out a little flesh without cutting through the base.
Melt the butter in a pan and toast the kadaif for 3–5 minutes, stirring, until golden and crisp. Remove from the heat and mix with the pistachio cream.
Fill the banana channels with the pistachio-kadaif mixture. Cut the puff pastry into 1–1.5 cm strips and wrap them around each banana in a spiral.
Place on the prepared tray and bake for 20–25 minutes, until the pastry is puffed and golden. Let cool slightly, then finish with powdered sugar, crushed pistachios or extra pistachio cream if you like.