Begin by taking a microwave-safe mug and adding the all-purpose flour, granulated sugar, baking powder, ground nutmeg, ground cinnamon, and a pinch of salt. Use a whisk or a fork to mix these dry ingredients thoroughly until well combined.
Next, pour in the eggnog, vegetable oil, and vanilla extract into the mug. Stir the mixture until the batter is smooth and free of lumps.
Place the mug in the microwave and cook on high for 1 minute and 30 seconds, keeping an eye on the cake as it cooks.
Once the cooking time is up, carefully take the mug out of the microwave and allow it to cool for a few minutes.
Top the mug cake with a generous dollop of whipped cream and sprinkle a dash of nutmeg and cinnamon over it.