Preheat the oven to 375°F. Spray 10 (½-cup) ramekins with nonstick vegetable oil spray. Add ⅓ cup granulated sugar to one ramekin and tilt to coat the bottom and sides. Pour the extra into the next ramekin, tilt to coat, and continue with the rest of the ramekins, until all are coated. Set aside.
Combine the chocolate and butter in a small saucepan over low heat and stir just until melted and incorporated. Remove the saucepan from the heat.
Add the espresso powder to the chocolate and stir until it dissolves, transfer the chocolate mixture to a small mixing bowl and let cool to room temperature.
In the bowl of a stand mixer fitted with a whisk attachment, whip the egg whites at low speed until frothy. (Note: When beating egg whites, an impeccably clean bowl is a must; even a bit of grease can keep them from firming up to form soft or stiff peaks.) Increase the speed to medium and slowly add ½ cup sugar. Whip at medium-high speed for 3 to 4 minutes, until the whites hold stiff peaks. (Note: Egg whites at the soft peak stage will be glossy and foamy, retain their basic shape but will droop a bit, and won’t entirely cling to the bowl. Egg whites at the stiff peak stage will be glossy and very firm, will retain their shape and cling to the bowl. They will stand straight up from the overturned beater. Yet they will still be creamy and flexible enough to fold in with other ingredients.)
Transfer the whipped whites to a clean bowl and, using the same stand mixer bowl (no need to wash it yet) fitted, whisk the egg yolks and remaining ¾ cup sugar for 4 to 5 minutes, until thick and creamy and a lemon-yellow in color. Add the cooled chocolate mixture and mix until just incorporated.
In a mixing bowl, whisk together the potato starch, salt, and hazelnut flour. Add to the mixer and mix at low speed, stopping to scrape down the side of the bowl as needed, until just fully incorporated.
Remove the bowl from the stand mixer, and with a rubber spatula, fold one-fourth of the whipped egg whites into the chocolate mixture and stir gently. There will be a few wisps of white still visible. Fold in the remaining egg whites.
Gently scoop the mixture evenly into the ramekins and bake for 12 to 15 minutes. The cakes will remain somewhat moist in the center, but not wet or overly gooey.