1.Preheat oven to 180°C/350°F. Line oven trays with baking paper.
2.Beat butter and caster sugar in a small bowl with an electric mixer until light and fluffy. Beat in egg until combined. Stir in sifted flour and soda, and combined buttermilk and extract.
3.Spoon 1½ tablespoons of the mixture, about 5cm (2 inches) apart, onto trays. Bake about 15 minutes or until puffed and browned lightly. Transfer to a wire rack to cool.
4.Sift icing sugar into a small bowl; stir in glucose, juice and the water until smooth. Transfer half the icing to another small bowl; stir in sifted cocoa, adding a little water if necessary.
5.Turn cookies flat-side up; spread white icing over one half of each cookie. Spread chocolate icing over remaining half. Stand on a wire rack until set.
Glucose syrup is often quite difficult to measure as it is very thick and sticks to everything. To make it easier, warm the jar of glucose syrup in a bowl of hot water before spooning out the required amount. Lightly oiling the spoon, or heating the spoon under hot water, and drying, will also help you to spoon the syrup out of the jar. The biscuits will keep in an airtight container for up to three days.