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 blended.
02 - Pour in the milk, vegetable oil, and vanilla extract. Stir with a fork until a smooth batter forms, scraping the sides and bottom of the mug thoroughly.
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 yet still moist in the center. If needed, continue microwaving in 10-second increments until fully set, being careful not to overcook.
05 - Let the cake rest in the mug for 1 to 2 minutes before serving. Enjoy directly from the mug.