Peel and chop the mango in small, even pieces; sprinkle with a teaspoon of lime juice and reserve.
Halve the avocados and remove the pits; scoop the flesh into a medium bowl. Mash coarsely with a fork, then add the remaining tablespoon of lime juice, salt and garlic and mix. Stir in the scallions.
Spread the guacamole in a small serving bowl. Sprinkle mango on top of the guacamole, then dust with chili powder. Serve immediately with chips or quesadillas.