Method

• 1.
  To make the coffee syrup, place sugar, coffee and 1 cup (250ml) water in a small saucepan over high heat and bring to the boil, stirring until sugar dissolves. Remove from heat and stir in rum. Cool to room temperature before using.
• 2.
  To make mousse, place mascarpone and cream in a stand mixer fitted with the whisk attachment (see notes). Whisk on high for 3-5 minutes, until medium peaks (when you lift the whisk, cream should form peaks that bend). Chill until ready to assemble.
• 3.
  Meanwhile, heat rum in a small saucepan over medium heat and bring to just below a simmer. Squeeze excess water from gelatine and stir into rum until dissolved, then set aside at room temperature.
• 4.
  In a clean stand mixer bowl, whisk yolks and 1/2 cup (110g) sugar on high speed for 3-4 minutes, until pale and fluffy. Transfer to a large bowl. In a clean stand mixer bowl, whisk egg whites and extra 1/3 cup (75g) sugar on high for 5-6 minutes, until firm peaks and sugar has dissolved.
• 5.
  Using a whisk, gently stir gelatine-rum mix and pistachio paste into yolk mixture until combined. Then gently fold through cream mixture, followed by meringue, until combined. Chill until ready to assemble.
• 6.
  Whip extra mascarpone to stiff peaks and fold through 1 cup pistachio mousse. Transfer to a piping bag fitted with a 1cm star nozzle. Chill until ready to assemble.
• 7.
  To assemble, grease an 18cm springform pan and line sides with acetate (see notes) or double layers of baking paper to a height 10cm above edge of pan. Place on a large baking tray. Dip savoiardi, one by one, in coffee syrup and use to line base of pan. Spoon in one-fifth of the mousse, levelling it evenly with a spatula. Repeat 4 more times, alternating sponge fingers and mousse, finishing with a layer of mousse. Chill for 3-4 hours, until set.
• 8.
  Unmould cake, transfer to a cake stand and peel away lining. Pipe stars of pistachio mousse on top of cake. Decorate with a generous dusting of cocoa and a sprinkle of chopped pistachios.