Wash the blackcurrants and pick them over for spoiled fruit.  Stemming is optional.  Blackcurrant leaves and stems are often used for flavoring in many different foods, and they'll only add more tannin and depth to the cassis.

Place the blackcurrants, cinnamon stick and sugar in a quart mason jar.  Add the vodka.  The vodka may not quite fit in the jar, but fill it up as much as you can.

Cover the jar and store in a dark place for several months, shaking anytime you remember.

After 2 to 3 months, strain the mixture.  Bottle it up and allow it to mature for another few months.  Enjoy!