Preheat a grill to medium. In a small bowl, mash the avocados until smooth. Season with salt.
Using a food processor, finely chop the garlic. Add the tomatoes and chiles and process for 5 seconds. Season with salt.
Split the rolls, leaving a long edge attached, and spread the butter on the insides. Place the franks on the grill, cover and cook, turning, until lightly charred, 8 to 10 minutes. Meanwhile, place the open rolls butter side down on the grill, cover and toast until lightly golden, 3 to 5 minutes. Cut each frank in half crosswise and place a half in each roll. Spread the avocado mash down the length of each frank, then top with the tomato salsa.