Preheat oven to 375° F. Cut a 4 by 30-inch strip of parchment paper.
Generously butter a 9-inch springform pan. Disassemble the pan and wrap the parchment on the inside edge, pressing it to adhere to the sides. Put the pan back together carefully, working to keep the parchment smooth. Add a little butter behind the overlapping end, if needed, to help it lay flat. Butter the parchment paper and set the pan aside.
Combine the chocolate and butter in a medium heatproof bowl. Set the bowl over a saucepan of barely simmering water, stirring frequently until melted and smooth. Remove the bowl from the saucepan to cool slightly.
In the bowl of a standing electric mixer fitted with a whisk, mix eggs to combine. With the mixer on low speed, pour in the sugar and add the salt. Increase speed to medium-high and whisk until the eggs are pale yellow and fluffy, about 5 minutes.
Fold about 1/3 of the egg mixture into the warm chocolate mixture. Scrape the chocolate mixture into the remaining eggs and fold gently until evenly combined.
Transfer the batter to the prepared pan and bake until the top puffs up, turns crisp, and cracks, about 25 minutes.
Cool the pan on a rack until just barely warm to the touch. Place the cake in the freezer for several hours or overnight.