Preheat the oven to 275°F. Line 2 large baking sheets with parchment paper.

Bring 2 inches water to a simmer in a medium saucepan.

In a heatproof bowl that easily sits on top of the saucepan, whisk together egg whites, brown sugar, orange zest and salt.

Place the bowl over simmering water and cook, whisking constantly, until mixture is slightly thickened and very warm (155° to 160°F on an instant-read thermometer), about 5 minutes; make sure the water is simmering, not boiling, or the mixture may cook too quickly.

Transfer hot egg mixture to a stand mixer fitted with the whisk attachment and whisk on medium-high speed until it is cooled completely and forms stiff, glossy peaks, about 8 minutes.

Remove the bowl from the mixer.

Drizzle half of the chocolate over the top and fold in with a few gentle strokes.

Drizzle with remaining chocolate and fold until just swirled in.

Use a soup spoon to scoop up and drop about 1/4 cup batter at a time onto the prepared baking sheets, spacing cookies about 1 inch apart; if you like, use the back of a small spoon to even out each cookie and swirl top to form a peak. (Alternatively, pipe out cookies using a pastry bag fitted with a large plain or star tip.)

Bake until cookies are dried but still a little soft inside, about 45 minutes.

Turn off the oven, prop open the door and let cookies cool completely in the oven, about 1 hour.

Remove from the baking sheets and serve, or store in an airtight container at room temperature for up to 3 days.