In a microwave-safe mug, combine flour, sugar, cocoa powder, and salt.
Mix the dry ingredients thoroughly with a fork.
Add vegetable oil and milk to the mug.
Stir in the vanilla extract until well combined.
Fold in chocolate chips or nuts if desired.
Microwave the mug on high for about 1 minute.
Check if the brownie is cooked through; it should be soft but not liquid.
Let it cool for a minute before eating.
Optionally, top with whipped cream or ice cream.
Enjoy your delicious brownie right from the mug!