Preheat oven to 400 F.
Grease the inside of a springform pan. Cut a strip of wax paper so it lines the sides. Line bottom of springform pan with waxpaper as well. Wrap the springform pan in aluminum foil. Set into larger pan with 1 inch very hot water.
In large metal bowl set over a pan of hot water. Bottom of the bowl should not touch the water. Combine chocolate and butter. Let stand, stiring occasionally until smooth and melted. Remove when there are only a few lumps left. In double boiler, over simmering water, heat eggs, stirring occasionally to prevent curdling, until just warm to the touch. Remove from heat and beat at high speed with electric mixer until eggs are triple the volume and soft peaks form. (about 5 minutes) Beat till cool. Using rubber spatula fold half the eggs into the chocolate mixture until almost incorporated. (be gentle) Fold in rest of eggs again until just blended and there are no streaks. Gently scrape into prepared pan. Lightly grease a aluminum foil and place on top of pan. Place in oven and bake for 5 minutes. Take aluminum foil off and bake for 10 more minutes. Cool for 45 minutes in a draft free space. Refrigerate for 3 hours.