01 - In a microwave-safe mug (at least 12 oz), whisk together the gluten-free flour, cocoa powder, sugar, baking powder, and salt until evenly combined.
02 - Add the milk, vegetable oil, and vanilla extract to the mug. Mix thoroughly with a fork until a smooth, lump-free batter forms, scraping the sides and bottom to ensure even mixing.
03 - Fold in the chocolate chips if using, distributing them evenly throughout the batter.
04 - Microwave on high for 1 minute. Check for doneness — the cake should be set on the edges but still slightly moist in the center. If needed, continue microwaving in 10-second increments until just done. Avoid overcooking to maintain a fudgy texture.
05 - Allow the cake to cool for 1 to 2 minutes before serving directly from the mug.