Step 1: In a large microwave-safe mug (12-14 oz capacity recommended), combine the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, and salt. Whisk vigorously with a fork to remove any lumps and ensure ingredients are well combined.
Step 2: Add the milk, vegetable oil, and vanilla extract to the dry ingredients in the mug. Mix with the fork until just combined and smooth. Be careful not to overmix; a few small lumps are fine.
Step 3: If desired, stir in the chocolate chips or chunks for an extra gooey center. Gently spread the batter evenly in the mug.
Step 4: Microwave on high for 60-90 seconds. Cooking times vary significantly by microwave wattage, so start with 60 seconds. Check for doneness: the cake should look set around the edges but may still be slightly moist in the center. If needed, microwave in additional 10-15 second increments, being careful not to overcook, which can make it rubbery.
Step 5: Carefully remove the hot mug from the microwave using oven mitts. Let it cool for 1-2 minutes before digging in directly from the mug, or invert it onto a plate if you prefer to eat it with a fork.