Place a rack in the center of the oven and preheat it to 350°F (180°C).
Coat a 9-inch (23 cm) springform pan with butter or baking spray. Line the bottom with parchment and butter or spray the parchment, then dust the pan with cocoa powder. Place the pan on a rimmed baking sheet lined with parchment or a silicone baking mat. In a food processor, combine the nuts, 2 tablespoons (25 g) of the sugar, the chocolate, coffee, and cinnamon and pulse, scraping the sides and bottom of the bowl often and taking care that you don’t process for so long that the walnuts become a paste and the chocolate melts. You want to end up with a bread-crumb–like mixture—it’s better to have some discernible morsels than to overdo it.
Separate the eggs, putting the yolks in a large bowl and the whites in the bowl of a stand mixer or a large bowl that you can use with a hand mixer.
Working with a whisk, beat the yolks until they’re homogeneous. Gradually whisk in 3/4 cup (150 g) of the sugar and then beat until the mixture is pale and your whisk leaves tracks, about 2 minutes. Beat in the vanilla. Switch to a flexible spatula and stir in the walnut mixture.
Add the salt to the whites. Attach the bowl to the mixer stand, if using, and fit it with the whisk attachment. Beat the whites until they are foamy, opaque and just a bit thick, 1 to 2 minutes.
Beat in the remaining 1/4 cup (50 g) sugar, adding it a tablespoon at a time. Once all the sugar is in, the whites should be thick and glossy—lift the beater(s), and the meringue should hold a pretty peak. If it doesn’t, continue to beat for 2 to 3 minutes more.
Using a flexible spatula, scoop out about a quarter of the meringue and add it to the bowl with the egg yolk mixture. Stir everything together energetically so that the whites lighten the thick mixture.
Scrape the rest of the meringue into the bowl and, being gentle, stir and fold it in. (Without overdoing it, you want to get as much of the meringue into the nut mixture as quickly as possible. If there are a few white streaks, it’s fine (better to have streaks than to knock all the air out of the meringue.) Scrape the batter into the pan, swiveling the pan from side to side to settle the batter evenly.
Bake until the cake feels firm to the touch and has risen and a tester inserted into the center of the cake comes out clean, 40 to 50 minutes. (The rise might be higher around the edges, but the middle should lift too.)
Move the pan to a rack and let rest for 5 to 10 minutes, then run a table knife between the cake and the sides of the pan to release the cake. Remove the sides of the springform and allow the cake to cool to room temperature on the rack.