Preheat your oven to 425°F (220°C) and butter the inside of a small ramekin or oven-safe mug.

In a microwave-safe bowl, combine bittersweet chocolate and butter. Heat in 30-second intervals, stirring until smooth.

In a separate bowl, whisk egg yolk and sugar until thick and pale, about 1-2 minutes.

Fold the melted chocolate and butter mixture into the egg yolk and sugar blend. Add vanilla extract and salt.

Sift in the flour and gently fold until no flour streaks remain.

Pour the batter into the prepared ramekin, filling it about two-thirds full.

Bake for 12 to 14 minutes until edges are firm but center is soft. Check at 12 minutes.

Let cool for 1 minute, then serve immediately with toppings of choice.