Preheat the oven to 325°F (165°C) and butter an 8- or 9-inch square baking dish.

Melt the butter and chocolate together in a heatproof bowl set over simmering water, stirring until smooth. Remove from heat.

Whisk in the sugar, then the eggs one at a time, then the vanilla. Add the flour, salt, and instant coffee (if using) and whisk just until the dry ingredients disappear.

Pour the batter into the prepared dish. Place the dish inside a larger roasting pan, set on a partially pulled-out oven rack, and add hot tap water to the outer pan until it comes halfway up the sides of the baking dish.

Bake for 50 to 60 minutes, until the top is set and puffed but the center still jiggles slightly. Carefully lift the dish from the water bath and cool for 10 minutes.

Scoop into bowls and serve warm with vanilla ice cream.