Coat sliced pineapple in butter, if using. Grill on grill pan or actual grill until both sides are browning, about 5-7 minutes on each side.
Meanwhile, combine tequila, cilantro and lime juice in a glass jar or cup. Using a muddler (or end of a thick spoon or ice cream scooper if you don’t have one), carefully and slowly smash the cilantro with the tequila and lime juice until cilantro is somewhat smashed. Add jalapeño and mash a few more times.
Once pineapple is grilled and cooled, squeeze juices into the jar or cup then add the pineapple in as well. You can also strain liquid if you don’t like the cilantro pieces.
Add fresh lime slices. Then add the fresh pineapple juice and soda/water if using.
Serve over ice with extra pineapple as garnish.
You can always leave out the tequila for a still delicious virgin option!
The butter is optional but using it helps the pineapple caramelize a bit more for an even better flavor.
You can also just keep the pineapple fresh and squeeze the juice out or muddle.