Combine the gelatine and 100ml of the milk in a bowl and leave for 10 minutes.
Put the remaining milk, the cream, sugar, zest, vanilla pod and seeds in a heavy-based pan, and bring to the boil. Take off the heat, mix in the gelatine and milk and whisk to dissolve. Cool to room temperature, then strain into a jug and pour into 2 glasses or dariole moulds. Cover and chill for a minimum of 5 hours, or overnight.
In a small saucepan, cook the extra 2 tablespoons of sugar and the seeds and juice from the passion fruit for 2 minutes until syrupy. Remove from the heat and allow to cool.
Serve the panna cottas in their glasses with the sauce spooned over the top. Alternatively, turn them out onto a plate (dip the dariole moulds into a bowl of boiling water for 30 seconds to loosen before turning out) and serve with the passion fruit sauce.