Preheat oven to 350 F. Line a 9- or 10-inch cake pan with parchment paper.

Place a kettle of water on to boil.

Melt chocolate and butter in the top half of a double boiler (or in a metal bowl held over a saucepan of simmering water).

Remove from heat.

Add beaten eggs, a little at a time, mixing well after each addition, until smooth.

Pour mixture into the prepared cake pan.

Place pan into a roasting pan and carefully pour in hot water until it comes halfway up the cake pan.

Bake for 25 to 30 minutes, or until just firm to the touch and not spongy.

Remove from oven and set aside to cool for 15 minutes.

Slide a knife blade around the edge of the cake, invert onto a plate, and remove pan and parchment paper.

Allow cake to cool completely.

Sprinkle confectioners’ sugar over the cake and serve immediately with whipped cream or ice cream, if using.