1.Split the vanilla bean in half lengthways; scrape out the seeds. Discard pod or save for another use (see notes).
2.Beat butter, sugar and vanilla seeds in a medium bowl with an electric mixer until light and fluffy. Beat in egg. Stir in 2¹⁄³ cups of the sifted flour. Knead dough on floured surface until smooth.
3.Divide dough in half. Return half the dough to the bowl; stir in essence and remaining sifted flour. Tint pink with colouring.
4.Divide the vanilla and strawberry doughs in half. On a floured surface, shape each portion of dough into 30cm (12-inch) logs (you will have two vanilla and two strawberry logs). Twist one vanilla and one strawberry log together to create a marbled effect; smooth into a neat log again. Repeat with remaining logs (you will have two marbled logs). Wrap logs in baking paper; freeze about 2 hours or until firm.
5.Remove logs from freezer; set aside for 15 minutes to soften slightly.
6.Meanwhile, preheat oven to 180°C/350°F. Line oven trays with baking paper.
7.Cut logs into 1cm (½-inch) slices; place about 3cm (1¼ inches) apart on trays.
8.Bake biscuits about 13 minutes. Stand 5 minutes on tray before transferring biscuits to a wire rack over another oven tray. Cool.
9.Stir chocolate in a small heatproof bowl over a small saucepan of simmering water until melted. Spoon chocolate into a plastic resealable bag; snip off one corner and pipe chocolate into spiral or zigzag patterns on biscuits. Stand at room temperature until chocolate is set.
Place the unused vanilla pod in a jar of caster sugar to make your own vanilla sugar.
These biscuits will keep in an airtight container for up to a week.