Add the butter to a mug, and microwave for 30 to 40 seconds, or until melted.
Once the butter is melted and smooth, add the sugar, milk and vanilla. Whisk until smooth.
Add the flour and salt, and whisk until a batter forms.
Microwave the mug on high for one minute.
Check the cake by pressing gently on the top. It should spring back. If it doesn't, continue to microwave in 10-second intervals until it does.
Serve immediately.
If deserved, add a squirt or dollop of whipped cream.