In a microwave-safe mug, combine 3 tablespoons rolled oats, 1 tablespoon all-purpose flour, 1 tablespoon brown sugar, 1/8 teaspoon ground cinnamon, 1/8 teaspoon baking powder, and a pinch of salt. Stir well to mix dry ingredients evenly.
Add 1 tablespoon melted unsalted butter, 1 tablespoon milk, and 1/4 teaspoon vanilla extract to the mug. Stir until all ingredients are combined into a thick batter.
Fold in 1 tablespoon raisins evenly into the batter.
Microwave the mug on high for 60 seconds. The cookie should be set but still soft in the center.
Let the oatmeal cookie cool for 1-2 minutes before eating directly from the mug.