Remove the skin from the tamarinds and place the pulp in a pan. Bring the tamarind pulp to the boil with water and let it cook until the seeds separate from the pulp.
Strain the mixture and collect the liquid. Add sugar to the juice and then bring it back to a boil together. Stir occasionally to properly dissolve the sugar. Cook the mixture over medium heat until it thickens. This usually takes about 15 minutes. Let the syrup stand for a while to cool. You only need a portion of this for this cocktail. You can save the rest of the syrup and use it in another recipe.
Mix the salt and chili powder together on a plate.
Cut a small slit in the center of a citrus fruit and rub the pulp along the rim of the glass. Dip the rim of the glass in the chilli salt, leaving a small rim on the glass.
Place the tequila, tamarind syrup, lime juice and Cointreau in a cocktail shaker and fill with ice. Shake well until everything is cooled. Fill the glass without the ice.
Make a spiral of a citrus peel as decoration and garnish the cocktail with a few slices of jalapeno for a little extra zing.