I keep tweaking the proportions to try to hit the perfect balance of citrus, sweet, and booze. Work in progress, but here's my current preference:
1.5 oz tequila
1 oz lime juice
0.5 oz orange liqueur
0.5 oz simple syrup
Shake ingredients with ice and strain into a rocks glass with new ice.
Not massively different to my 2010 recipe, but incrementally better, I think.
For a spicy version, slice up a couple of jalapenos and add them to a large jar of tequila for a day or two before straining the infused booze and following the recipe above.
