Grease a 20cm x 30cm lamington pan. Line base and sides with baking paper, extending paper 3cm above pan edges.
Process Arnott’s Butternut Snap Cookie in a food processor until finely crushed. Add butter. Process until combined. Press over base of prepared pan. Refrigerate while making filling.
To make custard filling, combine cream, milk, sugar and vanilla in a small saucepan over a low heat. Stir until hot. Don’t boil. Remove from heat.
Whisk egg yolks and custard powder in a heatproof bowl until smooth. Gradually add hot cream mixture, whisking until smooth. Return to saucepan and continue to whisk over a medium to high heat until boiling and thickened. Remove. Pour over base.
Cover surface with plastic wrap. Refrigerate for 2 hours, or until firm.
Place chocolate and butter in a medium microwave safe bowl. Microwave on medium heat for 20 second bursts, stirring in between, until melted.
Spread chocolate over custard filling. Chill until set.