Start with the rhubarb syrup. Simmer the ingredients together for about 15 min.
Strain through a coarse-meshed sieve. Carefully squeeze the rhubarb to extract all the juice. Leave to cool. Shake the tequila, Cointreau, lime juice and rhubarb syrup.
Serve in a cocktail glass with salted edges.
Tip: For the best salt rim, rub the glass rim with a slice of lemon then turn the glass upside down in salt on a saucer.