Combine cream, sugar, and lavender in a pan and bring to a boil while stirring.
Simmer the mixture for 3 minutes over reduced heat, stirring constantly.
Remove from heat, stir in lemon juice, and let cool for 5 minutes.
Pour into dishes and chill in the fridge for at least 3 hours.
Beat butter and lavender together in a bowl, then cream in the caster sugar.
Work in the flour and bring the mixture together to form a smooth dough.
Roll dough into a cylinder 4cm wide and 18cm long, then chill for 2 hours.
Preheat oven to 160°C and line a tray with parchment paper.
Cut dough into 5mm discs and place on the tray.
Bake for 15 minutes or until edges turn golden-brown, then cool on a rack.
Serve the chilled posset with the lavender biscuits.