For the marinade: Combine the lime juice, garlic, oregano, 4 tablespoons salt, 1 1/2 tablespoons pepper, chili powder, and oil in a large bowl. Add the chicken and marinate for 1 hour. Preheat the grill. Lift chicken out of the marinade and grill on both sides until nicely browned and cooked through, to 165 degrees F on an instant-read thermometer.
For the chipotle mayonnaise: In a food processor, puree the chiles to a smooth consistency. Set aside 3 tablespoons of the chipotle puree. Cook's Note: The chipotle puree can be spicy, so add less than the 3 tablespoons to reduce the heat in the mayonnaise. The remaining puree can be frozen for another use.
Clean the bowl of the food processor and add the garlic and brown sugar and pulse. Scrape down the sides of the work bowl and add 1/2 teaspoon salt, lemon juice, mayonnaise, sour cream and the reserved chipotle puree. Process to combine thoroughly. Store the mixture in an airtight container for up to 3 weeks.
For the pesto mayonnaise: Combine the mayonnaise and pesto. Store the mixture in the refrigerator for up to 3 weeks in an airtight container.
Spread the mayonnaise on the cut-side of the top and bottom halves of the buns. Top with Maximus Minimus Grilled Chicken thighs and cilantro (the more the better).