Beat butter, caster sugar and two-thirds of the vanilla in an electric mixer until light and fluffy (5-6 minutes), scrape down sides of bowl and beat in egg. Sift over flour and baking powder, stir to combine, turn onto a lightly floured surface and knead lightly until a soft dough forms (don’t overwork). Form into a disc, wrap in plastic wrap and refrigerate until just firm (30 minutes).
Preheat oven to 160C. Roll pastry on a lightly floured surface to 3mm thick and cut squares with a 7cm scallop-edged pastry cutter (see note), then place on oven trays lined with baking paper.
Combine demerara sugar and remaining vanilla in a bowl. Brush tops of biscuits with eggwhite, scatter with vanilla sugar and bake until edges are light golden (12-15 minutes). Cool on trays, then store in an airtight container until required. Sablé will keep for 3 days.
Note If you don't have a square pastry cutter,
a round one is just fine.