In a microwave-safe mug (at least 8 oz capacity), combine the all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt. Stir well to evenly distribute the dry ingredients.
Add the milk, vegetable oil, and vanilla extract to the mug. Mix thoroughly until a smooth batter forms with no lumps.
Fold in the finely chopped apple pieces, ensuring they are evenly distributed throughout the batter.
Microwave the mug on high power for 90 seconds. Check the cake for doneness by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.
If the cake is not fully cooked, microwave in additional 15-second increments until done, being careful not to overcook as the cake will continue to cook slightly after removal.
Allow the mug cake to cool for 1-2 minutes before enjoying. Serve directly in the mug.