Combine the black beans, cumin, chipotle powder, garlic, 6 stalks of cilantro, water, and a pinch of salt in a small saucepan.
Bring the mixture to a boil, then reduce to a low simmer and cook, partially covered, for 20 minutes.
Remove the cilantro stalks and garlic and use a potato masher to break up the beans. Set aside to cool.
Meanwhile, smash the avocados with a fork in a small bowl until mostly smooth. Add the lime juice and season to taste with salt.
To assemble, spread half of the bean mixture into a small baking dish (8x6-inch or similar).
Top with half of the avocado mixture, half the yogurt, half the sliced scallions, and half the tomatoes. Sprinkle the tomatoes with a little sea salt, then repeat all the layers, finishing with the tomatoes.
Garnish the dip with chopped cilantro and serve with plantain chips.
