Preheat the oven to 350℉ and grease an 8'' or 9'' metal baking pan. Crumple a sheet of parchment paper and line the tin. If you have them, use metal clips to hold the paper to the sides of the pan.

In a bowl over a pot of simmering water, melt the chocolate (alternatively, you could do this in the microwave in short bursts of 20-30 seconds). Once mostly melted, add in the cubed butter, stirring until glossy and smooth. Set aside to cool.

In the meantime, place 2 full eggs and 4 egg yolks into a bowl. Whisk in ½ cup granulated sugar, the cocoa powder, vanilla, and salt. Add in the cooled chocolate (it's okay if it's slightly warm), and stir until combined. Set aside.

In another bowl, whip the remaining 4 egg whites with a hand mixer or stand mixer fitted with the whisk attachment. Once the egg whites are frothy and broken up, gradually pour in the remaining ½ cup of granulated sugar, waiting  until the sugar is incorporated after every addition.

Whisk the egg whites until you have achieved firm peaks; it should hold its shape but not be fully stiff.

With a rubber spatula, gently fold two large spoonfuls of meringue into the chocolate mixture until mostly combined. Carefully add the rest of the meringue to the chocolate mixture, folding gently until the batter is almost fully blended (a few streaks are fine).

Pour the batter into the prepared pan (do not tap it on the counter) and remove the metal clips. Bake the cake for 35-45 minutes, until it is puffy, cracked, and risen. The top should look dry and set, and the middle will slightly jiggle.

Allow the cake to cool in the pan before lifting it from the tin and serving. It will sink and crack the more it sits.