Step #1 In a food processor/blender, whirl black beans just until slightly chunky.
Step #2 Scrape into a bowl & stir in the egg yolks, cumin, green onions, chilies, vinegar, baking powder, flour, & salt.
Step #3 In another bowl, with a mixer on high speed, beat egg whites until they form soft peaks.
Step #4 Fold whites into black bean mixture just until incorporated.
Step #5 Gently stir in the queso fresco.
Step #6 Pour 1 tsp oil into a 10- to 12-inch nonstick frying pan over medium-high heat; when hot, drop batter in 1/4-c portions into pan & spread into 2-inch-wide cakes with the bottom of measuring c.
Step #7 Cook, until cakes are browned on both sides & firm to touch in the center, turning once with a wide spatula, about 6 mins total.
Step #8 As cakes are cooked, arrange in a single layer on 12- by 15-inch baking sheets in a 200° oven; cover loosely with foil wrap & keep warm up to 30 mins.
Step #9 Repeat to cook remaining cakes, adding more oil to pan as necessary.