In a microwave-safe bowl or large mug, melt the butter (about 30–40 seconds).
Stir in the milk, granulated sugar, brown sugar, and vanilla until smooth.
Add the flour, cocoa powder, and salt. Stir until just combined.
For an extra gooey center, press additional chocolate into the middle.
Microwave for 60–90 seconds depending on your microwave and desired texture.