1In a small saucepan, heat milk on medium until just below boiling. In a medium heatproof bowl, whisk essence, yolks, caster sugar and cornflour until pale and creamy. Gradually add hot milk in a thin, steady stream, whisking to combine. Return mixture to saucepan. Whisk on low until custard boils and thickens. Stir in butter. Transfer custard to a heatproof bowl. Chill until cold.
2Meanwhile, in a small heatproof bowl, combine icing sugar and boiling water. Place bowl over a small saucepan of simmering water (do not let base of bowl touch water). Stir until icing is smooth. Tint icing with pink food colouring. Remove from heat.
3Spread icing onto top halves of biscuits. Set aside. Transfer custard mixture to a piping bag fitted with a plain nozzle. Spread bases of biscuits with jam and pipe over custard. Top with strawberries and biscuit tops. Serve with pistachios.