You CAN store panna cotta in the freezer (the plain panna cotta), but I prefer not to. Frozen panna cotta can have an effect on the texture. Once frozen, transfer the panna cotta to the fridge and leave it overnight, so that the frozen parts can thaw out slowly and evenly. Even then, there is a chance that the texture of the panna cotta could change and may not be even. So please be mindful of that.