In a microwave-safe mug, combine flour, sugar, cocoa powder, baking powder, and salt.
Mix the dry ingredients until well combined.
Add milk, vegetable oil, and vanilla extract to the mug.
Stir until the mixture is smooth and free of lumps.
Fold in chocolate chips or nuts if desired.
Microwave the mug on high for 1 minute.
Check if it's done; if not, microwave in 15-second intervals.
Let the mug cake cool for a minute before enjoying.
Top with ice cream or whipped cream if desired.
Serve and savor the chocolate goodness!