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 - 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 nothing sticks.
03 - Gently stir 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 but still slightly moist in the center. If needed, continue microwaving in 10-second intervals until just done, being careful not to overcook.
05 - Allow the mug cake to rest for 1 to 2 minutes before enjoying directly from the mug. Top with whipped cream or ice cream if desired.