1.Preheat oven to 160°C/325°F. Grease oven trays; line with baking paper.
2.Beat butter and caster sugar in small bowl with electric mixer until light and fluffy; transfer to large bowl. Stir in the water, sifted flours and half the lavender, in two batches.
3.Turn dough onto floured surface; knead gently until smooth. Shape dough into two 6cm x 20cm (2¼ inch x 8 inch) rectangular logs; cut into 1cm (½ inch) slices. Place about 2.5cm (1 inch) apart on oven trays; sprinkle with demerara sugar and the remaining lavender.
4.Bake shortbread about 20 minutes. Stand 5 minutes; transfer to wire racks to cool.
Dried edible lavender is available from specialist cooking stores. Store in an airtight container for up to a week.Note