Place a large bowlin the refrigerator and chill for at least 15 minutes. Meanwhile, in another large bowl, stir together the espresso, boiling water and ⅛ teaspoon salt until fully dissolved. Add the condensed milk and heavy cream, then whisk until homogeneous.

Pour the cold evaporated milk into the chilled bowl. Using a hand mixer or a whisk, whip the milk until it resembles softly whipped cream, has doubled in volume and forms mounds when the beaters or whisk are lifted; do not whip to stiff peaks. Whisk about one-third of the whipped milk into the condensed-milk mixture to lighten it. Now, using the whisk, fold in the remaining whipped milk until the mixture is well combined; it will resemble frothy yogurt.

Transfer to a 1½-quart container and cover tightly. Freeze until the center is fully frozen, at least 6 hours; the timing will vary based on the shape of the container and the temperature of the freezer. To check if the center is frozen, dig into it with a spoon; it should be solid, without any liquid.

Remove the base from the freezer and let stand at room temperature for 5 minutes to soften slightly. Using a sturdy spoon or ice cream scoop, transfer to a food processor; return the now-empty container to the freezer to keep it cold. Process until the mixture resembles a milk shake, 1 to 3 minutes, scraping the bowl as needed.

Return the base to the chilled container. Sprinkle with the chopped chocolate and quickly fold a few times to combine. Lay a sheet of plastic wrap directly against the surface, then freeze until firm, at least 4 hours. The ice cream will keep for a couple weeks.