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. Mix thoroughly with a fork until a smooth, lump-free batter forms, scraping the sides and bottom of the mug to ensure nothing is left unmixed.
03 - Gently stir in the chocolate chips if using, distributing them evenly throughout the batter.
04 - Microwave on high power for 1 minute. Check for doneness — the cake should be set on the edges yet still slightly moist in the center. If undercooked, continue microwaving in 10-second bursts until just done, being careful not to overcook.
05 - Let the mug cake rest for 1 to 2 minutes before enjoying directly from the mug.