Preheat the oven to 375°F (190°C). Lightly grease two small ramekins or oven-safe cups and place them on a baking tray.
Melt the chocolate and butter together in a microwave-safe bowl in 20-second intervals, stirring each time until smooth. Alternatively, melt over a double boiler.
In a separate bowl, whisk the egg, sugar, and vanilla extract until slightly thick and pale (about 1–2 minutes).
Add the melted chocolate mixture to the egg mixture and stir gently to combine.
Fold in the flour and salt just until the batter is smooth and even. Do not overmix.
Divide the batter evenly between the two prepared ramekins.
Bake for 10–12 minutes, or until the edges are set but the center is still soft. The middle should jiggle slightly when gently shaken.
Let the cakes cool for 2–3 minutes. Serve warm directly in the ramekins or carefully invert onto plates.
Garnish with powdered sugar, berries, mint, or a scoop of vanilla ice cream for extra indulgence.