Grease a 20cm x 30cm lamington pan and line the base and sides with baking paper.

In a food processor, blitz the Arnott's Butternut Snap cookies until finely crushed. Add the melted butter and process until combined. Press the mixture over the base of the prepared pan and refrigerate.

Combine thickened cream, milk, caster sugar, and vanilla bean paste in a saucepan over low heat until hot but not boiling.

Whisk the egg yolks and custard powder until smooth in a heatproof bowl. Gradually pour in the hot cream mixture while whisking until well combined.

Return to the saucepan and whisk over medium-high heat until it comes to a boil and thickens.

Pour the custard filling over the cookie base. Cover and allow to set in the fridge for about 2 hours or until firm.

Melt together the dark chocolate and butter in a microwave-safe bowl in 20-second bursts, stirring in between.

Once melted, spread the chocolate over the set custard filling and chill again until set.

Lift the slice from the pan using the baking paper and cut into pieces.