Grease three large baking trays. Put the butter and lavender flowers into a bowl and beat together (this releases the lavender flavour most effectively)
Beat in the sugar and poppyseeds, then when combined, stir in the flour. When mixed thoroughly, knead a little with your hands until smooth.
Divide the mixture into two and roll into two 'sausages' around 6" long. Wrap in cling-film and chill in the fridge until firm.
Preheat oven to 160C. Cut each sausage into around 10 slices and place them on the baking trays, allowing a little room for spread. Bake for 10-15 minutes until the shortbread is pale golden brown at the edges. Lift off the baking tray with a fish slice and leave on a wire rack to cool completely before eating.