In a small bowl, or in the 10-ounce mug itself, whisk together the flour, sugar, baking powder, salt and pumpkin pie spice.
Mix in the butter, milk, pumpkin, egg and vanilla until combined.
Microwave for 3 minutes, checking halfway through. After 3 minutes poke with toothpick to check that it’s all the way done. Note: the cake will rise over the mug slightly while cooking, this is normal, it will deflate when done cooking.
Let sit to cool.
Top with powdered sugar and a swirl of whipped cream and a sprinkle of pumpkin pie spice if desired.
Notes
If you only have salted butter on hand, you can use that but just leave out the additional salt in the recipe.
Heavy cream is strongly recommended in this recipe but you can use a whole milk if that is all you have on hand. Lower fat milks or milk substitutes will produce a different outcome.
Make sure your baking powder is fresh. So many recipes don't turn out properly because old or expired baking powder is being used. Check that can!
For this recipe, I used a 10-ounce, microwave-safe mug.