Gooey Fudgy Brownie Mug Cake

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