In a large mug, mix together the flour, brown sugar, baking powder, and pumpkin pie spice.
Stir in the milk, pumpkin purée, vegetable oil, and vanilla extract until smooth, ensuring to scrape the bottom of the mug to mix thoroughly. Sprinkle chocolate chips or nuts on top, if desired, and gently press them into the batter.
Microwave on high for 60-90 seconds, or until the cake is set but still slightly moist on top. Cooking time will vary depending on your microwave. Allow to cool for a few minutes before serving.