1.Preheat oven to 160°C (140°C fan forced). Grease two oven trays, line with baking paper.
2.Whisk egg yolks, vanilla and half the caster sugar in a medium bowl with an electric mixer for 5 minutes or until mixture reaches the ribbon stage.
3.In a small bowl, whisk egg whites and remaining caster sugar until thick and glossy. Set meringue aside.
4.Sift flour over yolk mixture, stir to combine. (This is best done very gently with a large metal spoon to maintain the air in the mixture). Gently fold the meringue into the flour mixture, in two batches, until combined.
5.Transfer mixture to a large piping bag. Pipe fingers 2cm (¾-inch) wide x 8cm (3¼-inch) long, and 2cm (¾-inch) apart on trays. Lightly sift over icing sugar.
6.Bake 25 minutes or until fingers are crusty on the outside. Turn oven off, leave fingers to cool in oven with door ajar.
7.Make choc-malt butter. Melt chocolate. Remove from the heat and beat with the remaining ingredients in a medium bowl with an electric mixer until smooth.
8.Spoon choc-malt butter into the medium piping bag, pipe onto a sponge finger, then sandwich with another finger. Repeat with remaining fingers and filling.
When beating the egg yolks and sugar, the ribbon stage indicates when you’ve dissolved the sugar and properly aerated the mixture. To check, remove a spoonful of the mixture, drizzle it over the top of the remaining mixture in the bowl; it should hold its shape briefly, before disappearing into the mix. You need a large piping bag fitted with a 2cm (¾-inch) plain tube, and a medium piping bag, fitted with a 1cm (½-inch) tube.