Preheat oven to 350°F (175°C). Grease an 8-inch springform pan, line the bottom with parchment, and wrap the outside tightly with a double layer of foil. In a heatproof bowl over simmering water, melt 8 oz chocolate and 1/2 cup butter, stirring until smooth. Remove from heat and let cool slightly.

In a large bowl, beat 4 eggs, 1 cup sugar, 1 tsp vanilla, and 1/4 tsp salt on high speed until thick, pale, and tripled in volume, about 5 to 7 minutes.

Fold the chocolate mixture into the egg mixture in three additions, using a rubber spatula, until no streaks remain. Be gentle to keep the batter airy.

Pour batter into the prepared pan. Set the pan in a roasting pan, place on the middle oven rack, and pour boiling water into the roasting pan to come halfway up the sides. Bake for 40 to 45 minutes, until the edges are set and the center still jiggles slightly.

Remove from the water bath and cool completely in the pan on a rack. Run a knife around the edge, release the springform, invert, and flip back onto a serving plate. Dust with cocoa powder or powdered sugar and slice.