Place the coffee and the caster sugar in a small saucepan and stir over medium heat until sugar is dissolved. Simmer for about 5 minutes until thickened and syrupy. Divide the syrup between the four ramekin dishes.
Place the coffee in a small bowl, whisk in the powdered gelatin and dissolve it. Stand for 2 minutes. Place the cream, milk, icing sugar and vanilla essence in a saucepan over medium heat and bring to a simmer, do not boil, stir to dissolve the sugar. Remove from the heat and add the coffee and gelatin mixture and whisk to combine. Allow to cool for two minutes.
Pour the Coffee Panna Cotta mixture over the coffee syrup in the four ramekin dishes. Refrigerate 4-6 hours until set. To release the desserts from the moulds, run a round bladed knife around the edge of the mould and quickly dip in a bowl of hot water. Invert over a serving plate.