Preheat oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.

In a large, heatproof bowl set over a saucepan of simmering water (a double boiler), melt the chopped chocolate and cubed butter. Stir frequently until the mixture is completely smooth and combined.

Remove the bowl from the heat and let it cool for 5-10 minutes. Whisk in the granulated sugar, vanilla extract, espresso powder, and salt until well combined.

Whisk in the egg yolks one at a time, ensuring each is fully incorporated before adding the next. The batter should be thick and glossy.

In a separate, clean, dry bowl, use an electric mixer with the whisk attachment to beat the egg whites on high speed until stiff peaks form. The peaks should hold their shape when the whisk is lifted.

Gently fold about one-third of the whipped egg whites into the chocolate batter to lighten it. Then, carefully fold in the remaining egg whites in two batches until just combined. Be careful not to overmix and deflate the whites.

Pour the batter into the prepared springform pan and gently smooth the top with a spatula.

Bake for 35-40 minutes. The edges will look set and the center will be slightly soft. The torte will rise and crack on top. A wooden skewer inserted into the center should come out with moist crumbs attached.

Let the torte cool completely in the pan on a wire rack. It will sink in the middle as it cools—this is expected. Once cool, carefully run a knife around the edge and release the springform collar. Serve at room temperature or slightly chilled.