Preheat the oven to 425 degrees F. Grease a 10-inch springform pan with butter and line the bottom with parchment
Melt the butter and chocolate in a heatproof bowl over a saucepan of simmering water on the stove.
In another bowl over the saucepan of simmering water, use a handheld electric mixer to beat the eggs until they are pale and thickened and form soft peaks. Fold the eggs into the chocolate in two batches.
Pour the batter into the prepared pan. Loosely cover with buttered foil.
Place the springform pan in a large roasting pan filled with of very hot water. The water should come halfway up the side of the springform pan. Bake for 5 minutes before removing the foil. Continue baking for 10 minutes until just set.
Take the pan out of the water and allow the cake to cool completely on a wire rack before removing from the pan.
The cake can be served at room temperature or chilled from the refrigerator.
Dust with confectioners’ sugar, if using.
Store in the refrigerator.
When the cake is served at room temperature, it is delicate and like a custard. After the cake is chilled in the refrigerator, it has a more solid and almost truffle-like consistency.