Preheat the oven to 325°F.

Line a 9-inch round pan with parchment paper, then grease the paper and sides of the pan. If using a springform pan, wrap bottom with foil to prevent water seeping into pan. Place the round pan in a larger rectangular pan (like a roasting pan). Bring one liter of water to a boil in a tea kettle or other pan.

Melt the butter and chocolate together. This can be accomplish with a double-boiler on the stovetop, or in the microwave. If microwaving, heat the chocolate & butter together in 30 second intervals, stirring in-between, until just melted.

Beat the eggs and salt together at medium-high speed for 5 minutes, until they have doubled in volume.

Fold ⅓ of the beaten eggs into the chocolate mixture, until almost incorporated. Gently fold in ½ the remaining eggs, then fold in the rest.

Scrape the batter into the prepared pan (which should be set in the larger rectangular pan), then place in the pre-heated oven on the middle rack.

Pour enough of the boiling water into the rectangular pan such that water comes halfway up the sides of the round pan.

Bake for 20-25 minutes. Edges should look just set, middle will still be jiggly.

Remove from the water bath, let the cake cool on a cooling rack, then refrigerate overnight (if this is not possible, try to at least freeze the cooled cake for 1 hour to allow the cake to set fully).

This will keep at least 3 days in the fridge, but it's never lasted that long for me.