In a microwave-safe mug, combine the rolled oats, all-purpose flour, brown sugar, baking powder, ground cinnamon, and salt. Stir well to mix all dry ingredients evenly.
Add the melted unsalted butter, milk, and vanilla extract to the dry ingredients in the mug. Stir thoroughly until a thick cookie dough forms and all ingredients are well incorporated.
Fold in the raisins evenly throughout the dough.
Microwave the mug on high power for 45 to 60 seconds. The cookie should rise and look set but still moist. Do not overcook to avoid dryness.
Carefully remove the mug from the microwave (it will be hot). Let the oatmeal cookie cool for 2 minutes before enjoying directly from the mug.