Step 1

Adjust oven rack to middle position and preheat oven to 390°F.

Step 2

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); alternatively, melt chocolate and butter together in a heatproof bowl in the microwave, stirring after every 20 second-interval. Once fully melted, whisk in powdered sugar (it's okay if it looks a little bit lumpy).

Step 3

Separate the egg into two small bowls. Whisk the egg white briefly with a fork, then add half the egg white to the yolk. Discard the remaining half of the egg white.

Step 4

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.

Step 5

Pour the batter into a 4-ounce ramekin. Bake until the cake is set around the edges but still molten within, about 12 minutes. Let cool for 1 minute before adding toppings. Serve immediately.