In medium saucepan, combine cream (150ml), butter (20g), brown sugar (30g), granulated sugar (50g), and salt (a pinch). Bring to a simmer over medium-low heat. Simmer 45 seconds.
Add chocolate (20g), and whisk to dissolve. Remove from heat, add cocoa (50g), and whisk until no lumps remain.
Return pan to low heat, and simmer sauce until glossy, whisking constantly, 20 seconds. Remove from heat, and stir in vanilla (1/4 tsp`). Serve warm. To reheat sauce, warm in a saucepan over low heat or microwave, stirring constantly. Do not boil.