Preheat oven to 350 degrees. Lightly grease the bottom and sides of a 9-inch round cake pan with nonstick cooking spray. Line the bottom with a round of parchment or wax paper, grease lightly and set aside.
In the top of a double boiler set over simmering water, combine the chocolate and butter, whisking until melted and blended. In a bowl, whisk together the eggs and sugar until well-blended. Gradually whisk in the chocolate mixture until blended. Pour the batter into the prepared cake pan. Place the pan in a larger baking pan. Fill the larger pan with enough hot water to come halfway up the sides of the cake pan.
Cover the larger pan with foil and bake in the center of the oven for 90 minutes or until the cake appears set and when you touch its center your finger comes away clean. Remove the cake pan from the water, transfer to a rack to cool completely. Serve plain or with gently whipped cream.