Preheat oven to 375°F (190°C). Grease an 8 or 9-inch springform pan, line the bottom with parchment, and dust with cocoa powder.

Melt the 10oz dark chocolate and cubed butter in a double boiler over simmering water until smooth. Stir in espresso powder and let cool slightly.

In a stand mixer, whip the eggs, sugar, vanilla, and salt on medium-high for 5-8 minutes until pale, thick, and doubled in volume.

Using a spatula, gently fold the slightly cooled chocolate mixture into the whipped eggs until just combined.

Sift the cocoa powder over the batter and gently fold until no dry streaks remain.

Pour batter into the prepared pan. Bake for 25-35 minutes until the edges are set but the center has a slight jiggle. Cool in pan for 15 minutes before releasing.

For the ganache, place chopped chocolate in a bowl. Heat heavy cream just until simmering, then pour over chocolate. Let sit for 3 minutes, then whisk until smooth. Whisk in the softened butter.

Let ganache thicken for 5-10 minutes, then pour over the completely cooled cake, allowing it to drip beautifully down the sides.