1 1/2 sticks (12 tablespoons) unsalted butter or margarine, cut into small pieces, plus more for the pan
12 ounces bittersweet chocolate, chopped
6 large eggs
1/2 cup sugar
Pinch of kosher salt
Unsweetened cocoa powder, for dusting
Preheat the oven to 350 degrees F. Lightly butter the bottom and side of a 9-inch springform pan. Combine the chocolate and 1 1/2 sticks butter in a heatproof bowl. Place the bowl over a saucepan of simmering water (do not let the bowl touch the water) and stir until melted and combined. Remove the bowl from the saucepan and let cool slightly.
Combine the eggs, sugar and salt in a large bowl. Beat with a mixer on medium-high speed until pale and thick, 5 to 8 minutes.
Gently fold half of the melted chocolate mixture into the egg mixture until just combined, then gently fold in the rest. Pour the batter into the prepared pan and bake until the top is no longer shiny and barely jiggles and a toothpick inserted into the center comes out with only a few crumbs, 35 to 45 minutes. Transfer to a rack and let cool completely in the pan. Remove the springform ring and transfer to a platter; dust with cocoa powder.