Step 1

Preheat oven to 160°C. Grease 2 large baking trays and line with baking paper.

Step 2

Using an electric mixer fitted with a whisk attachment, beat butter, caster sugar, golden syrup and vinegar until pale and creamy. Add egg and beat until combined. Sift flour, cocoa and bicarbonate of soda over butter mixture.

Step 3

Fold in sifted dry ingredients until combined and mixture forms a firm dough.

Step 4

Working in batches, roll teaspoons of dough into balls. Place 4cm apart on prepared trays and use a lightly floured fork to flatten slightly.

Step 5

Bake for 10–15 minutes or until biscuits are crisp and begin to darken. Transfer to wire racks to cool.

Step 6

Vanilla butter cream: Using an electric mixer, beat butter until very pale and fluffy. Add vanilla bean paste and beat until combined.

Step 7

With mixer on a low speed, gradually add icing sugar and beat until combined. If mixture becomes too stiff, gradually add warm milk, a little at a time, beating until creamy.

Step 8

Spoon buttercream into a piping bag fitted with a round nozzle. Pipe icing onto half of biscuits and sandwich together with remaining biscuits of a similar size.