Preheat oven to 325 F. Line a 9-inch springform pan with parchment paper, and spray parchment paper with nonstick cooking spray. Set aside.
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 (see the note below 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.
Chocolate: You can use bars of baking chocolate or use good-quality bittersweet chocolate chips.
Butter: I use salted butter. If you prefer to use unsalted butter, increase the salt to ½ teaspoon kosher salt or ¼ teaspoon of table salt.
Kosher salt: Or a pinch of table salt.
Mixing: If you're using a hand mixer instead of a stand mixer, you'll need to mix the egg mixture on high. You'll also need to add additional mixing time to get the egg mixture thick enough as a hand mixer isn't as powerful as a stand mixer.