In a microwave-safe mug, combine the flour, sugar, baking powder, nutmeg, cinnamon, and salt. Whisk these dry ingredients together until well blended.
Add the eggnog, vegetable oil, and vanilla extract to the mug. Stir everything together until you have a smooth batter. Make sure there are no lumps.
Microwave the mug cake on high for 1 minute and 30 seconds. Cooking times may vary depending on your microwave, so watch closely as it cooks. The cake should rise and not be wet in the middle.
Once done, carefully remove the mug from the microwave (it will be hot!). Allow it to cool for a couple of minutes.
Top the mug cake with a generous dollop of whipped cream. Finish it off by sprinkling a bit of nutmeg and cinnamon on top for an added holiday touch.