Juice the lemon and the grapefruit and then sieve to remove pith and seeds.
Measure the vodka into a jug and add the grapefruit juice and lemon juice.
Sieve the powdered sugar into the mix, and stir until it is dissolved. If you don't mind your frozen cocktail popsicles being quite tart in taste, you don't have to add the sugar. But grapefruit can be quite sharp, and not everyone is a fan! For added sweet/savoury flavour and added fragrance, stir in a pinch of rosemary salt.
Pour the frozen cocktail mix into mould and freeze for at least 4 hours, or preferably overnight.
Remove the vodka greyhound frozen cocktail popsicles from the mould and serve.