Char the poblano peppers over an open flame or under a broiler until the skins are blistered and blackened. Place them in a bowl, cover, and let them steam for about 10 minutes. Peel off the charred skins, remove the seeds, and roughly chop the flesh.
In a saucepan, heat the vegetable oil over medium heat. Add the diced onion and minced garlic, sauté until soft and translucent.
Add the chopped poblano peppers to the saucepan and stir for a minute. Pour in the broth and let it simmer for about 5 minutes until the peppers are tender.
Transfer the mixture to a blender and blend until smooth. Season with salt and pepper according to your taste. Set aside.
Divide the masa into 16 equal portions and shape each one into a three-inch ball.
Heat a nonstick griddle or cast-iron skillet over medium heat.
Individually press each masa ball between plastic sheets using a tortilla press. If you don’t have a press, use a rolling pin to flatten them into ovals or 'huarache' style.
Transfer the flattened masa onto the preheated skillet and cook each side for about 2 minutes.
As the memelas cook, spread butter on each side and add a layer of mashed black beans to one side.
Remove from heat and top each memela with the prepared poblano salsa, crumbled queso fresco, and slices of fresh avocado.
