Passionfruit is ripe when the skin is wrinkly. Supermarkets usually sell passionfruit before they get to this stage, so if you pick some up with a smooth skin, pop them in the fruit bowl for a few days to fully ripen. The unripe passionfruit is really quite tart, but this sharpness mellows as it ripens, so it's definitely worth hanging on until your fruit is wrinkly to enjoy this passionfruit cocktail at its best.