Preheat oven to 325 F. Line a 9-inch springform pan with parchment paper, and spray parchment paper with nonstick cooking spray.

In a heavy saucepan over low heat, begin to melt butter.

When butter is partially melted, add chocolate.

Stir regularly until chocolate and butter are melted and combined.

Set aside to cool.

In a large bowl, beat eggs, sugar, vanilla, and salt on low for 30 seconds.

Increase speed to medium-high if using a stand mixer (or high if using a hand mixer), and continue beating for 7-8 minutes, or until batter is thick and pale yellow. This step is key, so please don't cut the mixing time short.

Stream in cooled chocolate mixture until combined. Batter should look like thick chocolate mousse at this point.

Pour batter into prepared pan.

Bake for about 70 minutes, or until a wooden toothpick inserted in the center comes out clean.

Let cool completely on a wire cooling rack.

Gently remove the springform pan ring.

Dust with cocoa powder or powdered sugar, if desired.