Pour the vodka into a large, sterilised Kilner jar or similar container. Add the blackberries and the sugar, then seal tightly and give the jar a good shake. Leave in a cool, dark place for 3 days to allow the flavours to infuse, shaking daily.
Strain the berries and vodka through a fine sieve lined with muslin or a new, clean, damp cloth. (It’s important that the fine debris in the mixture is removed from the liquid otherwise it will ferment and spoil the vodka.) You may have to do this a couple of times to remove all traces of the fruit.
Pour the vodka into sterilised bottles or jam jars and seal them securely. Keep chilled and consume within 8 weeks.
To sterilise glass jars or bottles, wash with hot, soapy water and rinse clean. Alternatively, put them through a hot dishwasher cycle. Dry in the oven for 1 hour at gas 1, 140°C, fan 120°C.
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