Line a baking tray with baking parchment or a silicone mat. It is important that the tray you use will fit in the oven. Alternatively, cut your baking paper in half, so that when it comes to baking you can transfer them more easily.
In a medium mixing bowl, mix together the butter and icing sugar until light and fluffy.
Add the egg yolks, one at a time, until all 5 egg yolks have been incorporated.
Mix in the vanilla extract, salt, orange zest, flour and cornflour until smooth. You may need to scrape down the sides of the bowl a couple of times with a rubber spatula.
Spoon the dough into a piping bag fitted with a large star nozzle and pipe 5cm (2 inches) lengths onto the baking tray leaving at least 1cm between each one.Alternatively, you can pipe rosettes. Place the tray in the fridge for at least 30 minutes and then preheat the oven to 180°C fan / 400°F / Gas 6.
Baking: Place the tray on the middle shelf of the oven for 10-12minutes and bake until the biscuits are a nice pale golden-brown colour.
Cool on the baking tray for five minutes, then transfer to a wire rack.
Once the biscuits have cooled, place the chopped chocolate in a medium-sized heatproof bowl. Melt the chocolate (or in the microwave for a few seconds) over a pan of gently simmering water, stirring occasionally. *See notes.
Once smooth and completely melted, dip the biscuits one at a time and place them back onto the baking tray. Then sprinkle with the crushed pistachios.