Place gelatine leaves in a bowl and cover with cold water. Stand for 5 minutes to soften. Squeeze out excess liquid. Place the buttermilk and 1 tbs caster sugar in a small saucepan over low heat. Cook, stirring, for 2-3 minutes or until the sugar dissolves and mixture is just hot (do not boil). Remove from heat. Stir in gelatine until dissolved. Transfer to a bowl. Cool, stirring occasionally, for 20 minutes.
Gently whisk 1 ctn yoghurt into the buttermilk mixture. Whisk in the remaining yoghurt and lemon rind until just combined. Divide among 4 lightly greased 125ml (1/2 cup) metal dariole moulds. Place in the fridge for 4 hours or until set.
Meanwhile, place the vodka, lemon juice and remaining sugar in a small saucepan over low heat. Cook, stirring, for 2 minutes or until the sugar dissolves. Bring to the boil. Simmer for 2 minutes. Add the blueberries. Cook, stirring occasionally, for 3-4 minutes or until the blueberries release their juices and are just soft. Transfer to a bowl. Set aside to cool completely.
Turn panna cottas onto plates. Spoon over vodka blueberry syrup.