Combine the masa harina and salt in the bowl of a stand mixer with the beater attachment. With the mixer on low speed, slowly pour in ¼ cup hot water until the mixture forms a dough. It should be moist and tacky but not sticky or dry. If it's sticky, sprinkle in a little more masa harina and mix to combine; if it's dry, pour in a tablespoon or so more hot water and mix to combine.

When the dough feels right, turn the mixer to medium-high speed, pinch off a small piece of butter, and drop it into the dough, beating until it disappears before dropping in another piece. When all the butter is in the dough, turn the mixer to high and beat for another minute, until the masa dough is very fluffy. Check to see if it has remained slightly tacky but not sticky or dry; beat in more masa harina or water if needed to correct it.

Using a small cookie scoop or large tablespoon, form the masa into eight-equal pieces. Pat each one into a 3-inch disk.

Pour 1 tablespoon of the oil into a large skillet over medium-high heat. When it shimmers, add as many of the masa disks as will fit without overcrowding. Cook until very lightly browned, 1 minute per side, then remove to a plate and let cool slightly. While they're still pretty hot but you can (barely) handle them, pinch the edges of the masa disks all around, turning them up to form a rimmed basket shape.

Pour enough oil into the skillet over medium-high heat to come up ½ inch. When the oil shimmers, add enough of the masa boats (sopes) to fit without overcrowding and fry until crisp, 2 minutes per side. Transfer to a paper-towel-lined plate.

Scoop 2 tablespoons of the black bean puree into each of the sopes, then top with feta, salsa verde, and dressed lettuce. Serve while hot.