Adjust an oven rack to the lower-middle position and heat the oven to 325 degrees. Line the bottom of an 6 or 7-inch springform pan with parchment paper or waxed paper and grease the sides of the pan. Wrap the outside of the pan with 2 sheets of heavy-duty aluminum foil and set it in a large roasting pan. Bring a kettle or pot of water to boil.

In a stand mixer using the whisk attachment, beat the eggs at high speed until the volume doubles (about 5 minutes).

Meanwhile, melt the chocolate and butter together. I put them in a glass bowl and put it in the microwave for 30 second intervals, stirring in between until the mixture is melted and smooth. Then fold about a third of the beaten eggs into the chocolate mixture using a large rubber spatula until no streaks of egg are visible. Then fold in another third of the egg mixture, and then the last third of the egg mixture.

Scrape the batter into the prepared springform pan and smooth the surface with a rubber spatula. Place the roasting pan on the oven rack and pour in enough boiling water to come about halfway up the sides of the springform pan. Bake until a toothpick inserted comes out clean, about 20-25 minutes. Remove the springform pan from the water bath and set on a wire rack; cool to room temperature.