In a 12-ounce microwave-safe mug, whisk together flour, cocoa powder, sugar, baking powder, and salt.
Add milk, oil, and vanilla extract. Stir until just combined—don’t overmix. Fold in chocolate chips if using.
Microwave on high for 60–80 seconds, until the cake is just set on top and still moist underneath. (Start checking at 60 seconds; microwave times may vary.)
Let cool for 1 minute before enjoying. Serve warm, straight from the mug, topped with ice cream or whipped cream if desired.
