Gather the ingredients for your apple mug cake.
Combine the dry ingredients in a small bowl and set aside.
Place the butter in the mug and cover the top with a microwave-safe paper towel. Microwave on high for 30 seconds.
Pour the melted butter, milk, and vanilla extract into the flour mixture. Stir just until smooth and fully combined.
For the filling, add the apples, brown sugar, flour, and cinnamon to the mug. Stir well to evenly coat the apples.
Microwave the mug on high until the apples are slightly softened, about 45 seconds.
Stir the apples again, then pour the cake batter over the top. Do not mix.
Microwave the cinnamon apple mug cake on high for about 90 seconds. To check if it’s done, insert a toothpick into the top. If it comes out covered in wet batter, continue microwaving your apple cake in 5 to 10-second increments until cooked through. Enjoy!