Step 1

Heat the oven to 350°F with racks in the upper- and lower-middle positions. Line 2 rimmed baking sheets with kitchen parchment. Measure out 71 grams (2½ ounces) of the chopped chocolate and set aside.

Step 2

In a medium saucepan over high, bring 1 inch of water to a boil, then reduce heat to a bare simmer. In a medium heatproof bowl, combine the remaining 156 grams (5½ ounces) chopped chocolate, the butter, cocoa and espresso powder. Set the bowl on top of the saucepan; make sure the bottom does not touch the water. Stir occasionally with a silicone spatula until the mixture is completely smooth. Remove the bowl from the pan and cool slightly; keep the saucepan and water over the heat.

Step 3

In the bowl of a stand mixer, whisk together the egg whites, sugar, vanilla and salt. Set the bowl on top of 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.

Step 4

Using a silicone spatula, fold about one-third 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.

Step 5

Drop the batter in 2-tablespoon mounds spaced 1½ inches apart on the prepared baking sheets. Bake until the tops have cracked but the interiors still look 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.