Add the flour, sugar, and cocoa powder to the mug. Stir until the color looks even and there are no dry patches.
Pour in the milk and oil. Stir slowly until a thick, smooth batter forms. It should look glossy and dark.
If using chocolate chips, stir them in or drop them into the center so they melt while cooking.
Place the mug in the microwave. Cook for about 1 minute. The top should look set, but the center may still look soft.
Let it sit for 30 seconds. This helps the cake finish setting and cool slightly.
Add powdered sugar or a little chocolate drizzle on top if you want.