To make ricotta cream; whisk ricotta in a bowl until creamy. Push through a sieve to remove lumps. Whisk in zest and icing sugar. Fold in cream until combined. Refrigerate for 1 hour or until ready to use.
To make tuiles; whisk eggwhites and icing sugar in a large bowl until smooth. Sift flour and cocoa over the top. Add butter and whisk until smooth. Stand for 20 minutes.
Preheat oven to 180°C. Draw two 8.5cm circles onto a piece of baking paper. Place paper onto a greased oven tray. Put 2 level teaspoons of mixture in the centre of each circle and spread out with the back of teaspoon to evenly cover circle.
Bake for 5 minutes. Remove tray from oven and, working as quickly as you can, run a small flat metal spatula under each tuile to remove from tray. Invert onto bench and wrap immediately around the handle of a wooden spoon – they will be very hot. Once tuiles cool and become crisp (which won’t take long), slip off spoon handle and set aside seam-side down (to prevent unrolling). Repeat before other tuile cools and cannot be rolled. If tuile is beginning to stiffen before you can roll it, return tray to oven for 30 seconds – 1 minute to soften. Repeat with remaining batter.
Put melted chocolate and nuts in two separate bowls. Dip one or two ends of tuiles in melted chocolate then dip in pistachios. Put on a tray covered in foil until set.
When ready to serve, fill a piping bag fitted with a 1cm fluted nozzle with ricotta cream and pipe in both ends of tuiles. Only fill what you need – keep remaining unfilled tuiles in a container for 2 days.
Tip: Tuiles can take a little practice – allow a few goes to perfect rolling and timing. If rolling isn’t working for you, rest hot soft tuiles over a rolling pin to cool for 5 minutes. Dip one end in chocolate and pistachios and serve as wafers with ricotta cream.