In a microwave-safe mug (at least 12 ounces), whisk together the flour, sugar, and pumpkin pie spice until evenly combined.

Add the pumpkin puree and milk to the dry ingredients, stirring until you have a smooth batter with no lumps.

Microwave on high for 60-90 seconds, watching carefully until the cake has risen and the top appears set but still slightly moist.

Let the mug cake cool for 1-2 minutes before enjoying, and top with whipped cream or a sprinkle of cinnamon if desired.