• Preheat oven to 425° F.
• Butter springform pan, line bottom with parchment paper, butter parchment paper, and wrap outside of pan with two layers of tin foil.

Prepare batter

• Place chocolate and butter in medium microwavable bowl.
• Microwave chocolate and butter, heating 15 seconds at a time and then stirring, until nearly all the chocolate and butter have melted.
• Remove from microwave and stir until fully melted.
• Place eggs in metal mixer bowl.
• Place bowl over pan of simmering water.
• Heat, stirring constantly to prevent curdling, until just warm to touch, approx. 5 minutes.
• Remove bowl from heat and beat with mixer, using whisk attachment, until eggs triple in volume and form soft peaks when whisk is raised, approx. 5 minutes.
• Using large wire whisk or flexible spatula, fold ½ eggs into melted chocolate and butter until almost incorporated.
• Fold in remaining eggs until just blended and no streaks remain.

Bake

• Scrap batter into prepared pan.
• Smooth mixture with spatula.
• Place springform pan in baking pan.
• Add at least 1" of very hot water to baking pan.
• Place in oven.
• Bake 5 minutes.
• Cover loosely with buttered foil and bake 10 minutes. Cake should look soft at this point.
• Cool cake in pan on rack for 45 minutes.
• Wrap the cake, still in the pan, with plastic wrap and refrigerate until very firm, approx. 3 hours.

Unmold and serve

• Cover flat plate, larger in diameter than cake, with plastic wrap.
• Wipe sides of pan with hot, damp towel.
• Run thin metal spatula around cake between springform pan and cake.
• Release side of springform pan.
• Place plastic-wrapped plate on top of cake.
• Invent.
• Remove bottom of pan and parchment paper.
• Reinvert onto serving plate.
• Serve at room temperate.