Line a baking tray with baking parchment or a silicone mat.
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 and flour 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. 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.