Place six ramekins (7.5cm/3in wide and 4cm/1½in deep) in a deep-sided roasting tray. Preheat the oven to 150C/130C Fan/Gas 2.
Put 150g/5½oz of the caster sugar in a clean, heavy-based saucepan and heat over a high heat without stirring until the sugar melts and a golden-brown caramel forms. If you need to move the mixture to prevent it from burning, swirl the pan from time to time instead of stirring. (Caution: Melted sugar is very hot. Take care when cooking with it.)
Remove the pan from the heat and carefully add two tablespoons of recently boiled water to the caramel - it will spit and splutter a little, so keep your face at a safe distance from the pan. Swirl the pan until the sugar and water have combined to form a runnier caramel, then pour a little into the bottom of each ramekin, tipping them each this way and that until the caramel has evenly coated the base and sides. Set aside.
Heat the milk, cream and lavender in a separate saucepan over a low heat until just simmering.
Meanwhile, whisk the eggs and the remaining 75g/2¾oz sugar in a bowl until pale and fluffy.
Pour the hot milk mixture onto the egg mixture, whisking continuously until smooth. Strain the mixture into a jug through a fine sieve, then pour the strained mixture into the ramekins.
Add enough boiling water to the roasting tray to reach two-thirds of the way up the sides of the ramekins. Carefully transfer the roasting tray to the oven and bake the crème caramels for 40-45 minutes, or until just set. Remove from the oven and set aside in the roasting tray until cooled to room temperature.
When the ramekins are cool enough to handle, remove from the roasting tray and chill in the fridge for at least an hour.
For the shortbread, increase the oven temperature to 180C/160C Fan/Gas 4. Grease a 30x20cm/12x8in baking tin with butter.
Beat the butter and icing sugar in a bowl until light and fluffy, using an electric whisk. Fold in the lavender, cornflour and plain flour until the mixture comes together as a firm dough.
Press the dough into the prepared baking tin using your fingertips to form an even layer of dough about 1cm/½in thick. Prick the dough all over with a fork and sprinkle with the caster sugar, if using.
Bake the shortbread in the oven for 12-15 minutes, or until pale golden-brown and cooked through. Set aside to cool in the tin for a little while, then break into pieces.
To serve, remove the crème caramels from the ramekins by running a knife around the inside of each, then pressing the knife gently into one side of the crème caramels, and inverting the ramekins onto upturned serving plates. Serve with pieces of the shortbread alongside.