Preheat oven to 325F. I use a baking stone.
Grease 9" springform pan and cover bottom with parchment paper. This is going in a water bath, and springforms don't do this well. There are a bunch of approaches here, but some good ones are to use an oven bag and then foil, silicone sleeves, or to use another pan just slightly larger than your springform. Aluminum foil should work if you had a wide enough roll, but mine isn't.
Beat eggs on high for ~5min; volume should ~2x.
Melt chocolate and butter in microwave. Mix smooth. Target 115F.
Boil full kettle.
Fold eggs into chocolate, 1/3 at a time.
Pour into springform pan. Put cake pan in roasting pan or baking sheet in oven. Pour kettle into outer pan.
Bake until center measures 140F, 25min.
Remove pan from bath, cool to room temp on rack. Refrigerate at
least a few hours, up to 4d.