Immediately before you add the mixture to your ramekins, rub a little oil around the inside (very little – you just want the sides to be greased, not with enough oil to pool). Once the panna cotta has set, sit the ramekin in hot (not boiling) water for 20-30 seconds. Press your finger to the top edge of the panna cotta to release the vacuum, then turn it upside down onto your plate and give it a little shake.