Adjust oven rack to middle position and preheat oven to 200°C/390°F (standard oven setting).
Melt chocolate and butter together in a heatproof bowl set over a pan of simmering water (make sure the water doesn’t touch the bottom of the bowl) or in the microwave. Once fully melted, whisk in the powdered sugar.
Separate the egg, allowing the egg white to drip into a small bowl and placing the yolk in another one. Whisk the egg white briefly with a fork, then add half the egg white to the yolk, so that there’s 1 yolk and half an egg white in the bowl. Discard the remaining egg white.
Add the yolk and half the egg white to the chocolate mixture and whisk until incorporated. Add flour and salt and whisk until the batter is smooth.
Pour the batter into a 4-inch ramekin. Bake for 11-13 minutes, remove from the oven and allow to cool for about 1 minute before adding desired toppings. Serve immediately.
Recipe by The Tough Cookie at https://thetoughcookie.com/2015/02/28/messy-molten-chocolate-cake-2/