Spray a 9-inch spring form pan with nonstick cooking spray. Set aside.

Place the chopped chocolate and butter in a large microwave-safe mixing bowl. Microwave on high in 30-second increments, stirring between each, until the chocolate is fully melted and smooth. Allow the mixture to cool at room temperature for 5 minutes.

Place the eggs, sugar, vanilla extract, and salt in a large mixing bowl. Beat with a mixer on high speed for 3-4 minutes, until the mixture is pale, foamy, and nearly doubled in volume.

Pour the egg mixture into the melted chocolate. Fold gently with a rubber spatula until the batter is just smooth and no yellow streaks of egg remain.

Pour the batter into the spring form pan.

Bake the cake on the middle rack of oven for 60-70 minutes, until it is set and opaque on top.

Check to see that cake is done. Remove from oven or add time as needed.

Remove the pan from the oven. Allow to cool at room temperature for 10 minutes, then transfer to the refrigerator. Chill for at least 4 hours before serving.

Run a knife around the edge of the cake. Release the sides of the spring form pan. Use a fine-mesh sieve to dust the top of the cake with a generous coating of powdered sugar and serve.