Preheat oven to 200°C. Grease and line 2 baking trays with baking paper. Place butter and 1 cup cold water in a small saucepan. Bring to the boil over medium heat. Gradually add flour, stirring constantly until mixture comes together. Transfer to a large heatproof bowl. Allow to cool for 10 minutes.

Using an electric mixer, beat butter mixture for 1 minute or until pale. Add eggs, 1 at a time, beating constantly until well combined. Spoon or pipe tablespoons of mixture onto prepared trays, allowing room for spreading. Bake for 15 minutes. Reduce oven to 180°C. Bake for a further 30 minutes or until puffed, golden and dry inside. Transfer to a wire rack. Allow to cool completely.

Place chocolate in a heatproof, microwave-safe bowl. Microwave, uncovered, on medium (50%), stirring every minute with a metal spoon until almost melted. Stir until smooth.

Using a wooden skewer, make 1 hole in the base of each profiterole. Grease and line a 22cm (base) springform cake pan with baking paper. Using a piping bag fitted with a 1cm-diameter nozzle, fill profiteroles with custard (see tip). Place profiteroles, in a single layer, over base of prepared pan. Drizzle with half the chocolate and sprinkle with half the pistachios. Repeat with remaining profiteroles, custard, chocolate and pistachios. Refrigerate for 1 hour or until chocolate has set. Serve.