Whiz the shortbread in a food processor until finely ground. Add butter and whiz to combine. Press into a 35cm x 11cm tart pan and chill for 15 minutes or until firm.
Beat the cream cheese, mascarpone and 1 tbs lavender sugar with electric beaters until combined, then spread in the pan and chill for 1 hour or until set.
Meanwhile, place the jam, half the blueberries and remaining 1 tbs sugar in a saucepan over medium heat with 2 tbs water. Cook, stirring, for 2-3 minutes until the berries start to burst, then stir in the remaining berries. Mix the arrowroot with a little cold water in a small bowl, then stir into the berries and cook for a further 30 seconds or until the sauce is thickened. Remove from heat and cool completely.
Remove the cake from the pan and serve with the blueberry sauce.