Combine chipotle cooking sauce, beef broth, and 1/4 cup chopped cilantro in a saucepan; bring to a boil. Reduce heat to medium-low and simmer, stirring occasionally, for 10 minutes.
Heat oil in a skillet over medium-high heat; saute onion until softened, about 5 minutes. Stir garlic into onion and cook for 1 minute. Add roast beef and 1/4 cup chipotle sauce mixture and cook, stirring constantly, until heated through, about 2 minutes.
Ladle remaining chipotle sauce mixture into 4 bowls for dipping. Spoon roast beef mixture onto the bottom half of each bun, top with a cilantro sprig, and place the top on each bun. Dip sandwiches into sauce.