In a microwave-safe mug, combine the flour, sugar, baking powder, cinnamon, nutmeg, ginger, and salt. Mix well with a fork or small whisk until thoroughly combined.
Add the pumpkin puree and milk to the dry ingredients in the mug. Stir until the batter is smooth and there are no lumps.
Microwave the mug cake on high for 1 minute and 30 seconds to 2 minutes, or until the cake is set and springs back lightly when touched. Cooking time may vary depending on your microwave's wattage, so adjust accordingly.
Carefully remove the mug from the microwave (it will be hot!) and let the cake cool for a minute or two.
If desired, top the pumpkin mug cake with whipped cream, chopped nuts, or a sprinkle of cinnamon sugar.
Enjoy your warm and delicious pumpkin mug cake straight from the mug with a spoon!