In a cocktail shaker, muddle the blackberries and the jalapeño slice with the simple syrup.
Add the tequila, Cointreau and lime juice to the shaker. Fill about ¾ full of ice.
Shake vigorously for about 30 seconds to chill and dilute.
Run the lime around the edge of a cocktail glass and dip in salt (optional.) Fill with ice.
Strain cocktail into glass and garnish with blackberries and lime wheel.
Use fresh squeezed lime juice. Cut a wheel of the lime from the center to use for garnish and then squeeze juice from the rest of the lime. One slice of jalapeño (barely ¼ inch wide) provided just the right amount of heat for me.