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Preheat the oven to 350F (175C).
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Butter a 9-inch (23 cm) springform pan* and dust it with cocoa powder, tapping out any excess.
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If you suspect your springform pan isn't 100% water-tight, wrap the outside with aluminum foil, making sure it goes all the way up to the outer rim.
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Melt the chocolate and butter in a double boiler (or microwave), stirring occasionally, until smooth.
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NOTE: Chocolate can be bittersweet or semi-sweet and the butter can be salted or unsalted.
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Remove from heat.
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In a large bowl, whisk together the eggs and sugar, then whisk in the melted chocolate mixture until smooth.
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Pour the batter into the prepared 9-inch springform pan and cover the top of the pan snugly with a sheet of foil.
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Put the springform pan into a larger baking pan, such as a roasting pan, and add enough hot water to the baking pan to come about halfway up to the outside of the cake pan.
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Bake for 1 hour and 15 minutes.