Steps
In a microwave-safe mug, melt the butter and dark chocolate together in the microwave for about 30 seconds.
Stir until smooth.
Add sugar, egg, flour, cocoa powder, and salt to the mug, and mix well until fully combined.
Microwave on high for 30-40 seconds.
The cake should rise and look set on the edges but remain soft in the center.
Let it cool for a few seconds before digging in to enjoy the molten chocolate center!