Photograph: Brett Stevens
For the coffee caramel sauce, put the 200g sugar in a heavy-based saucepan over a medium heat. Leave, stirring occasionally, until the sugar caramelises and takes on a rich golden colour. Be careful: the hot caramel can spit so wear gloves.
Take off the heat, pour in the 100ml coffee, then stir over a low heat until the sauce is smooth. Leave to cool. Soak the gelatine leaves in cold water.
Mix the 300ml espresso and the cream in a bowl, then pour a quarter of this liquid into a pan and place over a low heat. Stir in the sugar and, once dissolved, remove the pan from the heat.
Squeeze the excess water out of the gelatine and add to the pan. Stir until the gelatine has dissolved, returning the saucepan to the heat if necessary.
Strain through a sieve (to remove any undissolved gelatine) into the remaining coffee and cream. Stir to combine, then divide between 6 x 200ml tea cups or ramekins and leave to cool.
Cover and chill for a few hours until set. To serve, spoon the sauce over the panna cottas.
Get ahead: you can make these the day before; chill. Keep the coffee caramel sauce at room temperature.