01 - Whisk together all-purpose flour, light brown sugar, baking powder, ground cinnamon, ground nutmeg, and salt in a 12–14 ounce microwave-safe mug until evenly distributed.
02 - Add pumpkin purée, milk, vegetable oil or melted butter, and vanilla extract to the mug. Mix with a fork or small whisk until the batter is smooth and fully combined.
03 - Microwave on high power for 1 minute 30 seconds to 2 minutes, until the batter rises and is set in the center. Avoid overcooking to maintain moistness.
04 - In a small bowl, blend softened cream cheese, powdered sugar, and pinch of cinnamon until smooth. Spoon the mixture onto the warm muffin, if desired.
05 - Enjoy immediately, warm, directly from the mug.