Preheat oven to 180°C (350 °F).
Lightly grease and line the base and side of 23 cm (9inch) spring-form cake tin with baking paper.
In medium saucepan, melt the butter over low heat then take of the heat, add in chopped chocolate and stir until melted.
In a small sauce pan pour in water and add half of granulated sugar. Cook until sugar is dissolved and syrup has thickened a bit, around 4-5 minutes, add in espresso powder, stir and remove from heat.
Pour hot syrup over melted butter and chocolate and stir until combined.
Separate 3 of the 5 eggs, placing the egg whites and yolks in separate medium-sized bowls. In the bowl with with yolks add the vanilla extract, cocoa powder, salt and remaining eggs. Whisk well to combine. Slowly add the cooled chocolate and butter mixture into the yolk mixture. Whisk well and set aside.
Using an electric mixer with whisk attachment whisk the egg whites until very frothy, the add in rest of the sugar and whisk until stiff peaks form.
Using a rubber spatula, gently fold the beaten egg whites into chocolate mixture until combined, making sure there are no visible streaks of egg white.
Pour the batter into the prepared pan and smooth the top.
Bake until the top is slightly cracked and the middle no longer moves, about 30 to 40 minutes, depending on your oven. (Cover the top with aluminum foil after 25 minutes if top starts to burn.)
Leave to cool completely before removing from the pan.
The cake sets as it cools and it MUST be completely cooled before being sliced.To be sure it is completely cooled, pop it in the fridge for 20-30 minutes before serving!
Dust with cocoa powder or top with whipped cream and chopped chocolate.