Heat the oven to 350°F with racks in the upper- and lower-middle positions. Line 2 baking sheets with kitchen parchment. Measure out 2½ ounces (½ cup) of the chopped chocolate and set aside.

In a medium sauce pan over high, bring 1 inch of water to a boil, then reduce heat to maintain a simmer. In a medium bowl, combine the remaining 5½ ounces chopped chocolate, the butter, cocoa and espresso powder.

Set the bowl on the saucepan over the simmering water (the bottom of bowl should not touch the water) and let the mixture melt until completely smooth, stirring often with a silicone spatula. Set aside to cool slightly; keep the saucepan and water over the heat.

In the bowl of a stand mixer, whisk together the egg whites, sugar, vanilla and salt. Set the bowl on the saucepan over the simmering water and, while whisking constantly, heat the mixture to 100°F. Attach the bowl to the mixer fitted with the whisk attachment and whip on medium-high until the mixture holds soft peaks when the whisk is lifted, 3 to 4 minutes.

Using a silicone spatula, fold ⅓ of the egg white mixture into the chocolate mixture until almost completely combined. Add the remaining egg whites and fold until a few streaks of white remain. Add the reserved chopped chocolate and fold gently until no white streaks remain.

Drop the batter in 2-tablespoon mounds spaced 1½ inches apart on the prepared sheets. Bake until the tops have cracked but the interiors still looks moist, 12 to 14 minutes, switching and rotating the sheets halfway through.

Cool on the baking sheets for 10 minutes, then transfer the cookies to a wire rack to cool completely, about 30 minutes.