Using a mortar and pestle, or on a cutting board using flat part of a knife, mash together garlic and salt until a paste forms. Scrape paste into a small bowl and stir in butter, lemon juice and cayenne. Taste and make sure salt and lemon are balanced.
Preheat grill. Place clams directly on grill grate or on a large baking pan in a single layer. Cover grill and let clams cook for 2 minutes. Open grill and check clams, using tongs to remove any that have opened, and transfer them to a large bowl. Be careful not to spill clams’ juices when transferring them. Close grill lid and check every 30 seconds, removing clams as they open.
Pour lemon-cayenne butter over clam bowl and toss lightly. Serve hot, garnished with chives.
