Add flour, sugar, cocoa powder, baking powder, and salt to a 12 oz or larger mug. Stir the dry ingredients together.
Add milk, oil, and vanilla extract to the mug. Stir until just combined — no dry streaks remaining.
Fold in chocolate chips if using.
Microwave on high for 60–70 seconds. The top should look just set. Do not overcook.
Let rest for 30 seconds before eating.
Notes
Microwave wattage varies. Start checking at 60 seconds and add 10-second intervals as needed. Top with a scoop of vanilla ice cream or a drizzle of chocolate sauce for an extra indulgent treat.