Preheat your oven to 350°F and position a rack in the center. While the oven heats, sift the powdered sugar into a small bowl to remove any lumps—this ensures your icing will be smooth and drizzle-able later. Measure out all your dry ingredients (flour, oats, cinnamon, baking soda, salt) into one bowl and set aside. This prep work ensures everything is ready when you need it, preventing rushed mixing and uneven baking.

In a large mixing bowl, combine the softened butter with both the brown sugar and granulated sugar. Using an electric mixer or vigorous hand mixing, beat for 2-3 minutes until the mixture becomes light, fluffy, and noticeably pale. This creaming process incorporates air into the dough, which helps the cookies rise and achieve a tender crumb. I always use room temperature butter because cold butter won't cream properly and warm butter will make the dough greasy.

Add the room temperature egg and vanilla extract to your creamed butter mixture. Mix on medium speed for about 1 minute until everything is well combined and the mixture looks smooth. The room temperature egg incorporates more easily and creates better emulsification with the butter base. I find that using Madagascar vanilla instead of regular vanilla really elevates these cookies with a richer, more complex flavor.

Add the dry ingredient mixture from Step 1 to your wet mixture from Step 3, then stir until just combined. Mix on low speed to avoid overworking the dough, which can lead to tough cookies. Once the flour is mostly incorporated, add the milk and continue mixing until you have a cohesive dough. The milk adds just enough moisture to help bind everything while keeping the dough tender.

Gently fold the plump raisins into the dough using a spatula or wooden spoon, being careful not to overmix. Once the raisins are evenly distributed, drop rounded tablespoons of dough onto your prepared baking sheets, spacing them about 2 inches apart to allow for spreading. I like to use a cookie scoop here for consistent sizing, which helps all the cookies bake evenly.

Bake the cookies at 350°F for 8-10 minutes, until the edges are lightly golden but the centers still look slightly underdone. Remove from the oven and let the cookies cool on the baking sheet for 2-3 minutes (they'll continue to cook slightly from residual heat), then transfer them to a wire cooling rack. Baking them slightly underdone ensures they stay chewy in the center; they'll firm up as they cool completely.

While the cookies cool, whisk together the sifted powdered sugar from Step 1 and milk until you reach a smooth, pourable consistency. Drizzle the icing over the cooled cookies using a fork or squeeze bottle, then let it set for 10-15 minutes before serving. The icing should be thin enough to drizzle but thick enough to stay on the cookies.