Bake pancetta slices until crisp, reserving the fat from the pan. Braise collard greens in a pot of salted boiling water, then cook with onions, pancetta fat, and stock in a pan until the stock has evaporated. For the espresso aioli, whisk mustard, egg yolk, and salt in a bowl, then slowly pour in oil and espresso while whisking to emulsify the aioli. To make the marbleized eggs, crack two eggs into a buttered pan and gently swirl yolks into whites, taking care not to scramble them. Add chives and salt, then assemble the sandwiches, spreading aioli onto the biscuits and dividing pancetta, collard greens, and eggs between them.