Preheat the oven to 350°F (175°C). Butter the bottom and sides of a 9-inch (23-cm) springform pan, dust it with a bit of cocoa powder, and tap out any excess.

In a large heatproof bowl, combine the chocolate, butter, and espresso. Set the bowl over a pan of simmering water and stir occasionally until the mixture is melted and smooth. Remove the bowl from the heat and stir in the vanilla.

In a stand mixer fitted with the whip attachment, whisk together the egg yolks and the ¼ cup (50 g) granulated sugar on medium-high speed until the mixture is light and creamy, about 1 minute.

In a clean, dry bowl and with a clean whip attachment, whisk the egg whites on low speed until they begin to hold their shape. Add the remaining 2 tablespoons (30 g) granulated sugar and whisk on high speed until the whites hold soft peaks.

Fold the beaten egg yolks into the melted chocolate mixture, then fold in half of the whipped egg whites. Fold in the remaining whites, mixing just until there are no visible streaks of egg whites. Don’t overfold.

Scrape the batter into the prepared pan, sprinkle with cocoa nibs, if using, and bake until the cake feels as though it’s just barely set in the center, about 25 minutes.

Let cool completely. Run a knife around the sides of the cake to help loosen it from the pan. Release the sides of the pan and dust the cake with powdered sugar, if using.