Step 1Make the tequila-lime butter. Beat the butter, jalapeno and lime zest in a bowl until creamy. Beat in the lime juice and tequila. The mixture should be very soft, but there should be no liquid showing; if you want, add a little more tequila. Beat in the cilantro and salt. Taste and adjust the seasoning. The mixture should be very forceful (it will be much milder when spread over the corn), but it should be evenly balanced between lime, butter and salt. Add more salt, lime or tequila as needed.
Step 2Spoon the butter onto a sheet of plastic wrap in the shape of a log and roll it into a cylinder. Twist the ends of the plastic wrap in opposite directions to firm the cylinder and roll it gently on the counter to eliminate any air pockets in the center. Refrigerate until ready to use. (Recipe can be prepared to this point a couple of days in advance).
Step 3Soak the ears of corn in water to cover for at least 30 minutes before grilling.
Step 4Grill the corn over a medium-hot fire until the kernels inside are golden and tender, 20 to 25 minutes. Remove the husks and most of the silk will come with them; rub off what little might remain. Slice the butter into discs and pass alongside the hot corn.