Preheat your oven to 350°F (175°C) and lightly grease two ramekins or small baking dishes.
In a medium bowl, whisk together melted butter, Nutella, and brown sugar until smooth.
Add the egg yolk and vanilla extract, mixing until fully combined.
Stir in the flour, cocoa powder, and salt until just mixed and smooth.
Divide the batter between the prepared ramekins.
Bake for 18–20 minutes, or until the edges are set and the centers are still a bit gooey.
Let cool for a few minutes before serving warm with ice cream or whipped cream if desired.