Steps
1. Preheat your oven to 350°F (175°C).
2. Melt the butter in a microwave-safe mug or bowl.
3. Stir in granulated sugar and brown sugar until combined.
4. Add the egg yolk and vanilla extract, mixing well.
5. Sift in the flour, cocoa powder, and salt; fold until just combined.
6. Stir in chocolate chips if using.
7. Transfer the batter to a small greased baking dish or ramekin.
8. Bake for 15-18 minutes, or until the center is set but still fudgy.
9. Let cool for a few minutes before enjoying directly from the dish.